Another possibility is that she has some sort of stomach ailment in the context of the fanfic. Maybe she ate something bad earlier and now her stomach is protesting. Or perhaps she has a condition that causes her stomach to make noises at unexpected times, which can add an interesting plot twist to the fanfic as other characters might react to it in different ways.
One way could be to write something like 'The girl's stomach let out a low growl, like a tiny beast awakening inside her.'
Once upon a time, there was a girl named Lily. She was in the middle of a very important meeting. Suddenly, her stomach growled loudly. Everyone in the room stared at her. She blushed deeply and tried to come up with an excuse. She said that she had a very hungry pet mouse in her pocket, which made everyone laugh.
A girl may have a pale white stomach if she is constantly stressed. Stress can affect the body's hormonal balance and blood flow, which may lead to paler skin. Also, if she has a genetic predisposition to having fair skin in that area, it will be pale white. Moreover, if she recently recovered from an illness, especially one that affected her digestive system, her stomach might look paler as the body is still in the process of fully recovering and getting back to normal blood flow and pigmentation in that area.
One reason could be that she's been really busy and hasn't had time to eat. For example, if she's a student cramming for exams all day long, she might forget to eat and her stomach would growl as a result.
Maybe she ate something bad like spoiled food or food she was allergic to.
Maybe Naruto is just teasing Neji. In the Naruto world, friends often playfully taunt each other. Naruto might have seen Neji do something that he thought was more like what a girl would do, like being overly cautious or having a particular grace in his movements. It could also be a random joke that Naruto came up with to get a reaction out of Neji.
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.
It could be due to a viral or bacterial infection. In a fanfic setting, perhaps she caught a bug from someone else in her fictional world. For instance, if she was in a crowded place like a concert or a busy market and came into contact with someone who was sick, she could get an infection that causes diarrhea.
Another reason might be as a form of punishment or lesson. Athena, being the goddess of wisdom, could be teaching Percy something about seeing the world from a different perspective. By turning him into a girl, he might gain new insights into how females are treated in the world of the story, whether it's in the demigod community or among mortals.
One possible reason could be that the Houndoom has a strong bond with the girl. In many fanfics, animals or Pokémon can form deep emotional connections with humans. Maybe the Houndoom was rescued by the girl, and this licking is a sign of gratitude and affection.