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.
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.
Grover would be panicked. He's Percy's friend and he cares a great deal about him. He might start running around trying to find help, like looking for a healer or someone with magical healing abilities. He'd probably be muttering to himself about how he should have protected Percy better.
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.
In fanfiction, Percy could throw up because of intense emotional stress. Think about it, if he's faced with a situation where he might lose his friends or family, the anxiety could be so overwhelming that it affects his physical state. He might also be exposed to some kind of dark magic that has a negative impact on his body. And sometimes, fanfiction writers like to put their characters through really tough physical challenges, like running for days without rest, which could lead to exhaustion and vomiting.
It could be due to some sort of poison or magical spell. In the Percy Jackson universe, there are all kinds of creatures and magic that could cause such a reaction. For example, if he was exposed to a slow - acting venom from a mythical beast, it might gradually make him faint.
Maybe he ate something bad in the fanfiction world. Like some enchanted food that didn't agree with him.
Perhaps it's due to the interference of the gods. The other deities might not approve of their relationship for various reasons. Maybe Zeus, fearing the power that could come from a union between Percy and Artemis, decides to put an end to it. In the world of Percy Jackson, the gods often meddle in the affairs of mortals and demigods, so this could be a factor.
He might leave because of a personal conflict. Perhaps he has a falling - out with some of the other campers or with the camp leaders. Like if he disagrees with a new rule or policy that the camp has implemented which he thinks is unjust.
Another reason might be that she is forced to leave. There could be a magical or divine force at play. Maybe a god or goddess, who has some grudge against Percy or his father Poseidon, manipulates the situation so that Sally has no choice but to abandon Percy. This would add a lot of drama and conflict to the fanfic.
Well, in some fanfictions, it might be related to his demigod powers. Since he has a connection to the water element, blood being a liquid could potentially be manipulated in a way similar to how he controls water. Maybe he can sense the water content in blood and use his will to move or influence it.