Three distinct possibilities come to mind immediately--
1. You are actually allergic to one very specific type of seafood and not to others-- and your reactions to "processed" seafood are because of cross-contamination with that very particular type, which isn't a problem when you are eating fresh caught stuff because the fishery doesn't overlap with where you are. I can eat Alaskan salmon just fine-- UNLESS it is processed with crustaceans. Perhaps you're allergic to a fish, not to a bivalve or crustacean.
2. You are allergic to something which is incidental to the processing of the seafood that you're eating. Have you checked labels for other ingredients? Have you ever had the problem with ANY other processed/restaurant food? Those might give you clues.
3. You are not specifically ALLERGIC to anything, but are extremely histamine sensitive-- leading to many of the same symptoms (but usually milder or non-life-threatening) as allergy-- especially hives, GI symptoms, and somewhat distinctly-- MIGRAINES. If you're unusually sensitive, ANY fish/shellfish products which are not extremely fresh could do this to you, and it might be somewhat random... you might be the only person affected in a group, too. The term that you'll want to take a look at is Scombroid Fish Poisoning.