This sounds like it could be kidney failure. DON'T BE TOO ALARMED BY THIS, as although it is eventually terminal, the disease is often quite slow in its progress and there is a lot that can be done in many cases to prolong and improve quality of life.
There's an excellent website about CRF (chronic renal failure) in cats, and a very active discussion list which you can find at
http://www.felinecrf.org/
This website has veterinary backing from some of the most eminent vets in the UK and abroad, and is run by some remarkable and immensely knowledgeable people, who will help you make the right decisions.
Good luck - you've done exceptionally well to keep your old lady this long, so you obviously care very much about her.
Carole