Harry could also try some home remedies from the wizarding world. Maybe he would brew a cup of herbal tea made from magical plants that are known for their stomach - soothing properties. Or he could ask Hermione for help. She's always well - read and might know of some spells or charms that could ease the pain.
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.
Later, he could become depressed. Harry has always valued his friends highly, so losing them would be a huge blow. He might isolate himself in his dormitory, not participating in school activities as much. He would probably be constantly thinking about what went wrong and how to fix it.
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.
Harry could try to find out the root cause of the rejection. He might start investigating if there are any external influences on his mates. He could also reach out to other characters for support, like some of the Hogwarts professors. And he might try to prove himself to his mates, by showing them that he's still the same Harry they used to know deep down.
Harry would probably be deeply hurt. He might pour himself into his studies or his work in the wizarding world. He could also try to distract himself by taking on more dangerous tasks or missions. But ultimately, he'd have to face his feelings towards Ginny and decide whether to forgive her or move on completely.
He might focus on his studies. In the wizarding world, there are so many interesting magical subjects like Potions, Transfiguration. By burying himself in books and learning new spells, he can at least distract himself from the lack of friends.
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.
One way could be having him rest and drink some warm water to ease the pain.
In romance fanfics, if a character has a stomach ache, their love interest might take care of them. For instance, making them a special soup and staying by their side.
The demon could influence Harry by whispering dark thoughts in his mind. For instance, it might make him more cynical about the wizarding world and its values.
First, think about the cause of the stomach ache. Is it something he ate? Stress? Or an illness? Then, develop the characters' reactions. The boyfriend might be in pain and a bit grumpy. His partner could be worried and nurturing. For example, you can write how she tries different home remedies like giving him ginger tea.