I don't have experience with the problems you're having, but I just wanted to add FWIW that the fact that she's eaten chicken for years without a problem doesn't necessarily mean that she's not NOW allergic to it. Many years ago I went through food allergy testing and ALL the things I was allergic to were things I'd eaten for years without a problem. In fact, I was told that it is the things we eat a lot of that we're most likely to become allergic to.
More recently, my cat suddenly started being sick after eating and the vet said it was possible that he had become allergic to the chicken he'd been eating for months and to feed him a different protein for a while, which we did, and he stopped being sick; after a while we gradually reintroduced the chicken and he was fine.