yeah because he was seeing visions of the guy who killed his parents, whilst being tortured by his teacher.
in the 4th, he was being a bitch about being in a tournament that he really didn't have to be in.
Think about it, he could have just not shown up, and gotten last in every event. Almost everybody didn't want him to compete, and he didn't really either. He even slept in for one of the events, and nothing happened to him, so I'm guessing there was no crazy curse on him or anything. It also would have saved Cedric, and Voldemort wouldn't be back in his human form.