The information he had read in the chapter: Werewolves explained this clearly. In short, a monthly transformation when there is a full moon and the person in the next few days is in poor health, as it is painful.
'No. I can't jump to conclusions...' thought James. He had to make sure of this first. The previous times Lupin disappeared, he didn't pay attention if they were full moon nights.
Still, it was a very valid theory, as it would explain why he disappears right on full moon nights and his subsequent condition.
'If the next month, he disappears on another full moon night, he's most likely a werewolf...' thought James as he went back to bed. Falling back to sleep cost a lot more, as he had a lot of thoughts in his head about Lupin.
If his theory is true, where did he go to become a werewolf? The forbidden forest? James doesn't believe in this possibility. It is almost impossible for Headmaster Dumbledore not to know about Lupin's condition. Besides, it would make sense since they say that Dumbledore is a very flamboyant wizard.
...
Monday, January 31, 1972.
In the Hogwarts infirmary, a brown-haired boy lay on one of the beds sleeping. His complexion was pale and he looked like he had had the worst night of his life.
His eyes opened with difficulty, revealing brown irises. The boy looked at his surroundings with confusion, "How did I end up here?" he asked in a low, doubtful tone.
"With Professor McGonagall, we brought you here after your transformation was over. It's best to sleep in a comfortable bed," said a soft voice next to her. It was Madame Poppy, the Hogwarts nurse. If any student heard her they would be surprised, as her tone was very kind, compared to her characteristic strict tone.
"Thanks..." said Lupin in a low tone, but before he could say anything else, he felt a great pain all over his body. He clenched his teeth tightly and endured it, anyway, he was used to it. He had had worse nights.
"What time is it?" asked Lupin in a weak tone. He didn't want to miss breakfast under anything in the world. The best way to regain his strength was to eat, and a lot of it.
"There's still time for you to eat breakfast... but I don't like the look of you. It's crazy to think you can start a whole day of school like this," said Madame Poppy, looking at Lupin's pale complexion and dark circles under his eyes.
She felt very sorry for this little boy, who was only in his first year of school. That's why she was so kind to him.
"I'm fine. I want to go" said Lupin a little uncomfortable. He wasn't used to adults who knew about his condition being nice to him. The only adults who treated him nicely were his parents.
Poppy sighed but said nothing. She made him take something before letting him go, a remedy that tasted cold and strawberry. Drinking it made Lupin feel better and regain some of his energy.
Lupin hurried to the Gryffindor tower to change into his uniform as quickly as possible. To his luck, when he entered the dormitory, none of the marauders were awake. He didn't want them to look at him strangely again and have to give explanations or rather excuses.
In a few minutes, he changed and went out again, his destination: the Great Hall.
When he reached the doors of the Great Hall, he heard a voice addressed to him with a hostile and mocking tone, "Where is your little group of idiots?"
Lupin turned his head with a frown. He wasn't in the mood, and even less so when he had to go through a full moon night. He noticed three boys. It wasn't Rabastan and his gang. It was other Slytherin boys.
He recognized only one of the three boys, Severus Snape. A thin boy with pale skin, a big nose, and greasy black hair. He recognized him because Snape was very arrogant in potions class.
Lupin didn't like the greasy-haired boy either. He had had a few run-ins with him in Potions class. Although Snape, looked like a scrawny loner with no confidence, his tongue was sharp and acidic, plus he was very good at potions (as much as Lupin didn't want to admit it which made him act arrogant.
The other two boys, one with very short black hair and one blond, he didn't know. All the first-year Slytherin students hated the marauders. He couldn't remember everyone's name.
After the flower prank, where most of the Slytherin's first years were affected, the hatred of the Slytherin boys towards their group increased, as James and Sirius made fun of them and were the main suspects.
As for the stink bomb prank, hardly anyone believes it was them. The only one was Rabastan, but again, he could do nothing to prove it.
Lupin huffed in annoyance and decided to ignore the boy, but the boy stepped in front of him, barring his way.
"What do you want?" asked Lupin, clenching his fists. His aggression was higher after a full moon night.
"I don't want anything. I just like it here," said the boy with a wicked grin.
When Lupin took a step to the side, the boy did the same, not to let him pass. Snape and the blond boy watched the scene with a faint smile amused.
"Bastard, move!" shouted Lupin angrily, something rarely seen from him. With great force, he pushed the boy who almost crashed into the door of the Great Hall. Luckily he managed to recover and regain his balance.
He, Snape, and the blond boy took out their wands. Lupin did the same. He was at a disadvantage, but at least he would take one with him. Thanks to James, he had learned useful charms for dueling.
"What's going on here?" said a male somewhat husky voice, coming from behind Lupin.
Turning his head, he could see a boy much taller than him. He looked like a wall compared to his lanky body. Lupin could recognize him immediately, William Turner. The captain of the Gryffindor Quidditch team, is one of the most popular people in the school.
Next to him also stood another member of the team, Alex Shaw, the team's seeker and seventh-year student. He was taller than William, but his build was slimmer but very athletic.
"Why are you blocking the way?" asked Alex in a cold tone, looking at the Slytherin trio in front of him.
"W-we were just about to go in," said the boy quickly as he turned and walked into the Great Hall, along with Snape and the blond. They were intimidated by the aura of two older and most popular students in the school.
"Are you alright?" asked William with a friendly smile compared to before.
"Yes... thanks," said Lupin, putting his wand away and trying to calm his anger.
"It's nothing. You're James' friend, right?" he asked, and Lupin nodded.
"If you're his friend, you're ours too" added William as he brutally patted Lupin's back and started walking.
'How much strength does he have...' thought Lupin in pain.
"By the way, where's James?" asked Alex, who seemed a bit more reserved than William.
"He's still sleeping..." replied Lupin a bit uncomfortable, as it was the first time he was talking to these two older boys. He also noticed a lot of stares when they entered the Great Hall, even more than when he was with James and Sirius.
"Would you like to have breakfast with us?" offered William.
Lupin was silent in thought, then after a few seconds, he agreed. They had helped him and they were James' friends, so he didn't see the point in turning them down.
Together with William and Alex, they sat in a place where there were several older students. Which made Lupin's breakfast more uncomfortable. He was the only first year.
"Who is he?" asked a red-haired girl looking at Lupin.
"He's a first year, a friend of James, his name is..." said William.
"Remus Lupin," added Lupin, noticing that William didn't know his name.
"They keep getting smaller and smaller," commented a stout boy.
"Shut up, don't bother the first-year boy," said a girl reprimanding him.
Lupin ate his breakfast amidst the conversations and laughter of the older students. Luckily after a while, they forgot him and he was able to have a quiet breakfast, although it was a bit uncomfortable.
Just as he finished breakfast, the marauders arrived. They greeted him normally and this time they didn't ask him anything about his disappearance.
'It's better this way...' thought Lupin. The only thing he felt was a weird look coming from James, but when he looked at him again, he was talking to Sirius as usual.
"Hey, James! Come here, sit down. We need to talk about quidditch!" shouted William, calling James, who had no choice but to sit with his captain.
William was a crazy quidditch enthusiast and knew a lot. The only person who could follow their conversations on the subject without getting tired of it was James, so they often spent the whole time talking about historical matches, tactics, etc.
"You're always bothering him with your quidditch. Leave him alone," said the redhead, pulling James towards her and starting to argue with William. James with a poker face was in the middle.
'He's even popular among the popular...' thought Lupin, looking at James, who started eating his breakfast as if he didn't care about the discussion of the older students.
Lupin took advantage of this moment and left. He still had some free time before the first class.
The day passed normally for Lupin. Only that his body felt more sore than usual. What most caught his attention and worried him was the look that James gave him. The moment he looked at him, the black-haired boy averted his gaze.
It was difficult for Lupin to hide his secret, especially from his dorm mates. He had prepared excuses beforehand, but if they were smart enough they might realize that something was wrong.
The one that worried him the most was James, as he proved to be very knowledgeable and seemed to be a walking encyclopedia. Whenever he was asked about a potion, a charm, or an ingredient, he always answered by being very precise and detailed.
'Did he discover me...? No, no, impossible' thought Lupin shaking his head. In the conversations they had that day with the marauders, James acted normally, even with him.
So, it should be his imagination. Luckily, after class, James had to go to quidditch practice, which from now on was earlier as it was getting dark faster.
After that, Lupin did homework, played the odd game with Toby and the others, and went to bed early, as he was very tired.