What Causes Nausea After Eating Fish?

Everytime I eat fish I get very nauseous within minutes after I'm done. Then end result is, I always end up vomiting. I don't have any other symptoms that would indicate an allergy, so why can't I handle fish ever? I am 32 yrs. old, 5ft. in height and 200lbs. Became allegic to pineapple during 1st pregnancy and have been ever since. (My sister had same incident w/ fruit) No other family history of allergies. Have always had normal BP and other than chronic migrains since age three, am in perfect health. Ate fish alot as a kid, started making me sick around the age 20 or so.