Oysters and pregnancy?

I asked this in pregnancy and didn't get any answers. Every year for my birthday my parents get oysters (them steam them) and last time i was pregnant (3 months) they made me vomit. After i threw up i felt fine and ready to eat some more. (i didn't) I'm scared i'm going to get sick again. I'm 6 1/2 weeks and haven't told anyone yet. Any one have this happen? Any tips on how not to get sick?