So the other day while I was on the treadmill watching a recently taped Dr. Oz (hey, I find it motivating to exercise during Dr. Oz. At least I'm doing one thing right!), he mentioned reactions to fish. It was during a segment on what to order and what to avoid in restaurants. The discussion was that fish served in a restaurant is often "expired", or ahem...a little past it's freshness date. Beware as these are often listed on the "specials" menu. It turns out that "expired fish" releases histamine, which can cause a reaction characterized by increased heart rate, rash and hives.
This occurrence leaves the diner with the impression they have a fish allergy.
The diagnosis is histamine fish poisoning. The symptoms are strictly related to consuming fish that has been improperly refrigerated for a period of time. The treatment is antihistamines, based on the severity of the reaction. Symptoms generally subside within 6 hours.
Of course fish allergy is real and presents with many of the same symptoms. Talk to a medical care provider if there are any concerns of a true fish allergy.