In a 'James Potter vomiting fanfiction' plot, there are several potential reasons. He could be suffering from a magical illness. Wizards can get sick just like Muggles, and some magical diseases might have vomiting as a symptom. He might also have overexerted himself magically. If he used too much magic in a short period, it could take a toll on his body and lead to nausea and vomiting. Another possibility is that he was in a situation where he was extremely frightened. Fear can cause the body to have a physical reaction, and vomiting could be one of them.
One possible reason could be that he was poisoned. In the wizarding world, there are many dark spells and potions that could cause such a reaction. Another reason might be extreme motion sickness if he was on a wild magical ride, like on a particularly unruly broomstick or a flying magical creature.
Well, perhaps he ate something bad. In the wizarding world, there could be all sorts of strange magical foods that might not agree with his stomach. Maybe he had a bad reaction to some experimental potion he drank. Or, if he was in a battle, he could have been affected by a dark spell that not only injured him but also made him vomit as a side - effect. Also, if he was under a lot of stress, it could cause his body to react in such a way. For example, if he was worried about the safety of his friends or family during a dangerous time.
Chapter 787, "Changing Fortune", is the clearest early answer because it turns Lu's situation into a concrete plot event. Early on, How could she know about these things?. By the end, He immediately stood up and went to the Scripture Pavilion..
Chapter 703, "Slap Oneself", is the clearest early answer because it turns Slap Oneself Wu's situation into a concrete plot event.
Chapter 467, "Your Boars Have All Run Back!", is the clearest early answer because it turns Shang Shuyue's situation into a concrete plot event. A useful checkpoint is the way the scene starts with He had recently been meditating on how to accomplish the tasks...
Chapter 447, "Going Viral on the Internet", is the clearest early answer because it turns Shen Lan's situation into a concrete plot event. The chapter turns that into something concrete by moving from Old Sir Jiang: "...." His face had grown even worse.
Chapter 471, "This is What Happens When You Abandon Sister Li!", is the clearest early answer because it turns Shining Transformation Trip's situation into a concrete plot event.
Chapter 356, "I announce, Lili is the best of the night!", is the clearest early answer because it turns Miss Su's situation into a concrete plot event. The chapter gives readers a real answer by opening on Seeing such a bustling scene, Su Yinwan who had just alighted from...
Chapter 603, "Met Uncle Beggar", is the clearest early answer because it turns Jiang Shi Xu's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Our family's ships have flipped, but it should be just two of them."...
Chapter 686, "Why Not Make Him Mute?", is the clearest early answer because it turns Luo Xingmu's situation into a concrete plot event. The chapter turns that into something concrete by moving from In the makeup room, Jiang Yan sneezed three times in a row.
Chapter 454, "Are these two people crazy?!", is the clearest early answer because it turns Dragon Ridge Island's situation into a concrete plot event. It opens with Shang Shuyue felt that following Jiang Li was the best decision she... and closes on Alright, next we'll go for training!" Yu Li: ?.
Chapter 616, "I'm Going to Save Sister Xiao Wan!", is the clearest early answer because it turns Bai Lu's situation into a concrete plot event. The chapter turns that into something concrete by moving from Since the storm had hit that morning, he had been preoccupied and out...