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I am sorry to hear about your girl's decline!
I would be most concerned about the possibility that she has an underlying metabolic disease like kidney disease or hyperthyroidism. These can quickly be ruled out with simple blood testing with your veterinarian.
The other problem that should be rule out especially in an indoor and outdoor cat is intestinal parasites. Your veterinarian should be able to help you rule this out by testing her feces.
I hope this helps!
As always, if you have further questions, please don't hesitate to ask them!