Allergies or something else?…

I have a cat with stomach issues and can’t figure out what the actual problem is. If anyone has had similar issues or has advice I would love some help.

When I adopted her in October 2024 she came to me eating a dry chicken based kibble. I tried to move her over to wet food (slowly) and she got sick (bad diarrhea). After that I moved her back to the dry chicken kibble and she got better. I really wanted her on a wet food so after a month I tried a different chicken wet food but she got sick again. So I went back to the dry chicken kibble but this time the diarrhea didn’t go away. I tried fortifila probiotics and it made the diarrhea worse. After a month of diarrhea we went to the vet and got a fecal test (negative) some digestive care chicken wet food. After eating one can she was horribly ill.

At this point I decided to try a novel protein. We went with RAWZ digestive support rabbit (with a built in probiotic fibre). A week or so later she was having normal bowel movements. Yay! So I figured it must be a sensitivity to chicken, since all of the other foods were chicken based. I was a confused how she was fine with chicken when I got her, as her dry chicken food didn’t make her sick at the beginning, but figured she must have developed a sensitivity from stress.

A month later I decided to add Instincts limited ingredient rabbit dry food to hopefully cut down on cost (I had budgeted for a good quality cat food before getting a cat, but that budget didn’t account for 10 dollars a day on cat food). But even with this new rabbit only food, feeding 1 tablespoon a day made her sick again!

So I’m at a loss. Is it a food sensitivity to chicken? Does she need a probiotic fibre in her food, and if so why did fortifilla make her sick? Why can’t she eat the other rabbit based food? It doesn’t make sense for RAWZ digestive care to be all she can eat. Some ingredient in it must be fixing the problem or there is some ingredient that is doesn’t have that makes her sick. I really wish I knew what the actual root of the issue is, because this food isn’t affordable for me in the long run…