Don't go near them again, Headtime, it's almost certainly the oysters. I ate some seafood one lunchtime, felt fine, went to a concert in the evening and didn't feel bad until the next morning when I woke up late and feeling like death, so the time delay you describe is normal. I was ill for five days and can only remember a doctor coming and giving me injections to try and stop the sickness - I thought he was an alien at the time. I had a form of botulism complete with halucinations and other symptoms you can probably imagine. You become allergic to foods on the second or subsequent occasions that you eat them and it sounds like this is what has happened to you.