It could also be that he ate some magical food that didn't agree with him. For example, he might have tried a new kind of magical sweet from Honeydukes that had a strange side effect on his digestive system. Another reason could be that he was under a minor curse. Some of the dark wizards or witches in the story might have cast a spell on him that caused his stomach to ache as a form of petty revenge or to distract him from something important.
Perhaps it was due to extreme stress or anxiety. Harry often had to deal with a great deal of pressure, like when he was preparing for a dangerous mission or facing off against Voldemort. This stress could have manifested physically in the form of a stomach ache, just like it can happen to ordinary people in real - life situations.
Well, it could be that he ate some bad ninja food. Maybe he ate a spoiled ration from a mission. You know, in the world of Boruto, they might not always have the freshest supplies on long missions.
One possible fanfiction could be about a boyfriend who has a stomach ache because he ate too much junk food at a party. His partner takes care of him, making him warm tea and getting him a heating pad. They spend the day cuddling while he recovers.
In the fanfic, it might be due to a ninja technique gone wrong. For example, if he was trying a new jutsu that affected his internal chakra flow and it ended up giving him a tummy ache. This could be a really interesting plot point as it shows the risks of experimenting with new powers.
Well, one story could be that a girl ate too much junk food at a party. She started having a stomach ache later. As her stomach was in discomfort, she couldn't hold in a fart. It was really embarrassing for her, especially when she was around her friends.
In some fanfics, a character might get a stomach ache due to eating something strange in a magical world. For example, in a fantasy fanfic, a character eats a glowing fruit not meant for humans and gets a severe stomach ache. Remedies could be a special potion made by a local healer.
Food is a common reason. For example, if she ate beans, which are known to cause gas, along with some spoiled food, it could lead to both stomach ache and farting. Maybe she had a meal at a roadside stall where the hygiene wasn't so good.
He might go to Madam Pomfrey in the hospital wing. She has all kinds of potions and remedies to help with ailments, so she could easily give him something to soothe his stomach.
Describe the physical sensations. For example, say things like 'A sharp pain stabbed at his stomach' or 'Her stomach churned uncomfortably'. Also, show the character's reaction to the pain, like doubling over or clutching their abdomen. And don't forget to mention any associated symptoms, such as nausea or a feeling of dizziness.
One possible cause could be over - exertion during a fight. Naruto is known for pushing himself to the limit. If he has been in a long and intense battle, it might put too much strain on his heart. Another cause could be some sort of poison or dark jutsu that specifically targets his heart function.
Maybe he was poisoned during a battle. The poison could be slowly affecting his body and causing a fever as his body tries to fight it off. In fanfiction, the poison could be something unique created by an enemy ninja with special properties.