If you're waiting for results of testing, I presume it wasn't skin prick which is the only reliable one. I've been told I was allergic to peanuts from blood tests, but I eat them all the time with no reactions.
You can get new allergies, and get rid of some allergies, at any time. I just had grass added to my shots a few years ago.
What type of reaction did you have? Was your reaction closing throat or hives, which are classified as "true allergy" or was it a reaction like vomiting or tachycardia or migraine headache which are more signs of a "sensitivity" problem?
With shellfish, it can be not the food at all but how it's treated. Not saying you have my sensitivity to sulfites, which are sprayed on fish and shellfish to preserve them, but other things happen to these foods also. Sorry, but there are no standardized tests for some things like sulfites. You probably don't have this problem anyway or you'd have reported a problem with lots of other foods. It's only an example.