Re: fish intolerance?
It could be a sensitivity to something they're using to process the fish. It might also be something leaching from the can lining? If you've eaten canned fish before without noticing this, then maybe your taste buds have changed recently.
I sometimes get that if I've recently been ill or are about to be ill, especially with canned foods. I've had canned peaches that have tasted peppery, other times they've even tasted fishy. Bleah! After a week or so, everything's back to normal.
Since you're only having a reaction to canned fish, not fresh, then I wouldn't think you're allergic.