Well, the main ingredient is raw fish, and raw fish is an ingredient you can't be too careful with, as raw fish and meat easily gets contaminated and then you can feel it. And of course, it could also be that you were not as well as you thought you were and then raw fish/meat can easily get the better of you. Normally when properly prepared sushi should not cause any serious issues....
(That or you just visited the wrong sushi bar, maybe...)