I would get her into your vet and, in the very least, have some blood work done. Kitties, even when in the middle of a steep decline, are expert at masking their distress. You sometimes have to go on the most subtle of clues and/or read between the lines. Given what you've shared so far, something doesn't seem right and the sooner you get a handle on what that something is, the better off you'll both be. Now with that said, with a cat that age, you may decide against any aggressive treatment if something is, indeed, wrong, but you'll at least know what you're dealing with and if there's anything you can do to make her more comfortable.