It could be a sign of a magical disease in the fanfic. There are many fictional diseases that can be created in the Harry Potter universe. For example, a disease that attacks the magical core of a wizard and causes various physical symptoms, including vomiting blood. It might also be related to a sacrifice or a powerful magic he is attempting. If Harry is trying to perform a very difficult and dangerous magic spell, it could take a toll on his body and lead to such a disturbing symptom.
Well, one reason could be that he was poisoned. In the world of Harry Potter, there are many poisonous substances. A fanfic writer might create a scenario where Harry is unknowingly poisoned, and this causes him to throw up blood as his body tries to fight off the toxin. Another possibility is that he has some sort of internal injury from a fight. If he got hit by a powerful spell or a physical attack during a battle, it could lead to internal bleeding which results in throwing up blood.
Perhaps he has been exposed to some sort of deadly and unknown substance in the mythical world. This could be a new type of monster venom or a strange plant that he accidentally ingested. The substance could be attacking his body from the inside, causing his internal organs to bleed, and thus he throws up blood.
It could be due to a dark curse or spell. Maybe he was attacked by a powerful dark wizard using a forbidden spell that affects his internal organs and causes bleeding.
Well, it could be related to some sort of poison. Perhaps he was poisoned by an enemy who managed to slip something into his food or drink. The poison could be slowly corroding his insides and making him cough up blood as a symptom. It might also be part of a plot by a new villain in the fanfiction world who is trying to weaken Harry in a more insidious way than the usual dark wizards.
Well, it could be a result of a dark spell cast on him by a Death Eater. Maybe they found a new and insidious way to harm him without leaving an obvious trace.
Maybe he ate something bad in the fanfiction world. Like some enchanted food that didn't agree with him.
Well, it could be due to some sort of magical potion gone wrong. Maybe he drank something that was supposed to give him extra strength but instead made his stomach turn. Another reason could be that he got poisoned during a battle with a monster. Some monsters might have venom or toxins that could make him sick to his stomach.
It could be related to a new and strange power he's trying to control. Percy is constantly evolving and getting new powers. If he tries to tap into a power that his body isn't ready for, it might have some adverse physical effects like vomiting. Like when he first started to understand his water - based powers, there could be something similar but more extreme with a new power.
Maybe in the fanfic, there was a huge misunderstanding. For example, perhaps the Weasleys were wrongly accused of something that endangered Harry's life or the lives of his loved ones. It could also be that there was some sort of dark magic at play that made Harry believe the Weasleys were his enemies.
Another possibility is internal injuries from a battle. Fanfiction often has intense battles that are not part of the original story. Harry could have been hit by a powerful spell during a fight that caused internal damage, which then makes him cough up blood.
One possible reason could be food poisoning. If a teacher ate something bad before coming to school, it might make them throw up. For example, if they had some expired dairy products for breakfast.