Q. Cat is 13 I changed her moist food 2 days ago, one side of her mouth is inflamed. Won't purr or meow, I've seen slight drooling, won't eat her treats.
A. Have her examined by your veterinarian. A complete oral exam may reveal periodontal disease. A bad tooth (or teeth) can cause the symptoms you are seeing. She may need a dental cleaning including dental xrays and possibly extractions. If she does need some dental work, it will require anesthesia so she will also need blood work to make sure she can undergo the procedure.