Dimethyl mercury, which is found in elevated levels in some fish (particularly fish high on the food chain), is extremely toxic. My understanding though, is that the symptoms take a long time (months) to manifest, and you have to eat a lot of fish. Eating a lot of sushi for lunch on one day isn't going to make you acutely symptomatic. Sounds like food poisoning, which sucks, but probably won't kill you.
Hope you are feeling better!