I figured out the grain allergy after the vet confirmed that the
cat had food
allergies without too much trouble, because I had given her Fancy Feast with gravy, which she loves. Well, gravy usually has wheat flour in it to thicken it, and if I'm sensitive to gluten foods myself, then I reasoned that cats could be, too. And, since cats are true carnivores by nature, then it seemed reasonable to me that grains weren't supposed to be part of the cat's regular diet.