you're probably allergic to chitin, which is a polymer of N-acetylglucosamine. You could also possibly be allergic to some protein bound within the chitin. The shells of most crustaceans and insects are made of chitin. When you eat those kinds of things you typically break and remove the shell. A soft shell crab has started growing a new shell even before molting, so there is actually a lot of chitin on its surface tissue even if it hasn't grown a totally hard shell yet. Humans don't make chitin, and we generally don't eat it--so some people are born with a defective gene for the enzyme phosphatidylinositol deacylase, and when they consume chitin their body treats is as an allergen and has a temper tantrum, even though it's basically just sugar.
It's stupid, but so is your DNA.