I had hives when I was a kid a few times and it was always allergy induced. All of my allergies are environmental, though.
It sounds like fish could be the culprit- sometimes the body wont react until after it's been exposed more than once. Have you tried benadryl or something similar? If that helps it clear up, I would assume it's allergy, if not virus. Not much help, I know.
Good luck trying to figure it out.
Michelle -mom to Katlyn 4/00 , Jake 3/02, and Seth 5/04