Why i do get stomach ache after eating fish?

Stomach ache: You may be allergic to fish See an allergist to get tested

Problem eating fish: Dietary triggers for GI symptoms are individualized; it's worthwhile keeping a food diary to see if correlates with certain foods. In general, gut stimulants (caffeine) & dietary fats often precipitate IBS-like complaints. About a third of IBS patients tolerate poorly dietary fiber sources. How your fish is prepared may be more important from the perspective of symptoms than that it is fish.