Eren causes himself gastrointestinal problems due to his intense desire to defeat Titans.
While part of the 104th Training Corps, Eren receives a diagnosis from the clinic that his gastrointestinal issues are due to his extreme desire to defeat Titans. He is so upset over not being able to fight Titans that his bunkmates throw him out into the hall to sleep.
Later on in class, Eren's stomach hurts bad enough he wants to go to the bathroom. Jean teases him about needing to take a dump. Mikasa helpfully stands up and announces to the class that Eren needs to go to the restroom, which Eren does not find helpful at all.
Rico takes the trainees to the top of the Walls where Eren is relieved to be able to use the cannons to shoot at Titans, but his aim is so bad that he does not hit any of them. His classmates call him an awful shot. Eren worries that the stress is going to open a hole in his stomach or cause him to go bald. Mikasa comforts him, saying that even if his aim is bad, he is bound to hit one eventually. She adds that even if he went bald it would not bother her.
To help Eren, Armin, Jean, Connie, and Reiner get him a sheet of bubble wrap with Titan faces drawn on each bubble. Eren is finally able to decimate dozens of Titans without upsetting his stomach. However he goes overboard with his enthusiasm, and Connie tells Jean they should leave because Eren is pissing him off.