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Hello, this is Dr. Jess. I am sorry to hear about your kitty! The more common diseases we worry about for an older cat who is eating lots but not gaining weight include hyperthyroidism, diabetes, and kidney disease. These diseases all require bloodwork to diagnose.
Is she having any vomiting or diarrhea? Does she seem to drink more water than she used to? Any urinary accidents?
Ok. With all three of those diseases we can see changes in urinary behavior, so that also makes me suspicious she could have one of these diseases. With thyroid issues we can also see some issue with the haircoat.
I would definitely make an appointment with your vet in the next week to have her examined and get some bloodwork done to check for these diseases.