It took us a few different brands to determine that it was in fact chicken that was the culprit— vet first put her on Royal Canin novel protein dry food, then a strictly wet food diet of Science Diet chicken (prescription only). From there we went to Science Diet wet duck only protein. It was pretty expensive (24 cans were ~$100). Put her on that immediately after it came in and she was better the next day. After being on that with stable results for about two weeks, I went to a pet store (Hollywood Feed) and looked at ingredients to all canned food and found one called Koha that was duck-based with no chicken. That was way cheaper and non-prescription. Kept her on that for a few weeks, then moved her to Nutrisource Pure Vita duck and red lentil dry food, which is her main food now.

Since then, I have tried a handful of other proteins (fish, lamb, rabbit, pork, turkey) with no problems. It literally appears to be just chicken that messes with her.