What is wrong with me when I eat this?

I was just wondering if I'm allergic to some seafood or if my stomach just cant handle it. I have eaten shrimp before and it had no effect on me but I only ate a couple pieces due to the fact I do not like it but every time I have ever went to eat at an all you can eat seafood place I have gotten sick. I would eat more than shrimp though. I would eat things like oysters, lobster, crab, squid, scallops, etc...Then about thirty to 40 minutes after eating it, the sickness settled in and I would throw up non stop, have diarrhea and fever. Not once but every time I have ingested seafood. I have not gotten sick eating shrimp or fish though so I am confused. Also I thought if you were allergic to something. It would kill you not just make you sick. Every time it has happened, I had to go to the ER because it would not let up until they gave me some type of meds. Not sure what they gave me though...Any ideas what would be causing this?