Have you ever been tested by an allergist aimee.x? I ate shellfish into my twenties with no noticeable issues, and then gradually developed an intolerance for all exoskeletal sea creatures, which are to this day still my favorite "fish" food (especially shrimp)... My intolerance is due to intestinal tract problems, which encompasses more than just shellfish, though. Not a true allergy. As Rob mentions, consult your docs, and they may have you do a diary of sorts, and/or refer you to an allergist and/or adjust your meds. My meds get adjusted just about every visit to the rheumy (every 3 months... sigh).
"There but for the grace of God, go I."
"... His mercy endureth for ever."