Sherry's right. Allergy tests aren't very accurate. Keep a food journal with everything you eat and drink, meds too, and your reactions. This will take a couple of months at least!
Just to confuse things further, with shellfish it can be the preservative that was used to prevent black spot, and not the shellfish itself. That's why the test might show negative. I can eat shrimp if I wash them thoroughly after shelling.
Do you react immediately? What is your reaction?
Some people have true allergies to foods, some sensitivities/intolerances which are not measurable on skin tests or blood tests.
If you have something like reflux (GERD) within a couple of hours it's easy to figure out what the trigger foods are. Migraines sometimes take up to 2 days to come on in response to eating a trigger food.