It could be the toxins found in the fish, or any spices/seasoning you used in it. Did you season it with morton’s table salt?
I haven’t eaten any fish on my diet, so I am unsure whether I’d get a reaction to it or not; you might just want to avoid it for another month to be safe.
I get a reaction to this horrible non-organic chicken at my lunch place because it contains antibiotics…
If you did use morton’s table salt, use natural sea salt in the future.