Fish and shellfish are known cause of food allergy in adults. You should visit allergologist and have skin tersts for allergy. Other symptoms are hives, lips/face swelling, throat itching, difficult breathing...together with diarrhea.
In meantime, just exclude all fish and shellfish from your diet (let's say for a week) and observe result. If symptoms improve, then add one type of fish back into the diet, and if all is OK, add another type of fish, but at least 48 hours apart from the first fish. And so on. This way you may identify exact fish/shellfish which you are allergic to. You may be allergic to morte than one type of fish though.
It is imperative your doctor investigates chronic diarrhoea. It is hard to know what the cause might be - try keeping a food diary and record your trips to the loo also. It may be a food intolerance, bacterial infection, irritable bowel syndrome or something else - your doctor should take a history and perform a physical exam, then perhaps run a blood test or order a colonoscopy depending on the results. Long term diarrhoea is not good for body or bowel. Find another doctor. Best of luck