HI this is DRB and Here is what I would do with your kitty.
1.First get her on a hydrolyzed diet such as feline prescription diet ZD
2. Put your kitty on Prednisone 5 mg twice daily to reduce inflammation and to improve appetite
3. Put your kitty on Famotidine 2.5 mg BID to reduce gastric acid
4. Put your kitty on cerenia Monday-Thursday of every week to reduce nausea. Holiday from cerenia Fri, Sat. Sun
The above works for 95% of kitties with what you have described.
Your kitty sounds like she has a protracted food allergy that has led to the detriment of her overall helath
I hope this helps. Let me know if you need additional help. Sincerely, DRB
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Please note that the information I provide is intended as general information only and not intended to replace the advice of your veterinary professional.