Hello, Sounds like you has a significant reaction the grilled sushi. But you mention the pain depends on the quantity you consume! Allergic reaction to foods do not (usually) depend on amount as little amount trigger reactins in a short time. If blood test (specific IgE) to fishes are negative, you will need a skin prick test to the particular fish as skin tests are more sensitive. If skin tests to all fishes you have reacted to are negative, you may have a condition called histamine intolerance which is sometimes related to the amount of food ingested (usually foods high in histamine such as eggs, chicken, tuna, chocolate etc). Occasionally a bad reaction (with stomach cramps, nausea, vomiting and feeling ill for several days) to fish is due to scombroid toxin poisoning i.e., due to eating expired canned fish or semicooked fish. No specific treatment is required but antihistamines help. If the stomach pain is due to several unrelated foods, check with a gastroenterologist to exclude ulcer disease.