If not stress, could be bladder related - crystals, cystitis although with the latter you would usually see at least some, possibly quite a lot of blood in the urine. I would be surprised if at that age she didn't have some kidney deterioration. Your vet will take a blood and a urine sample (although you may prefer to do this at home using something like Katkor as to have the vet take urine directly is not very comfortable for many cats). Make sure the vet does an SDMA test which is a very sensitive test for kidney disease. They should automatically offer it but surprisingly not many do. There is something that can be done for all of this except cystitis which usually resolves on its own, particularly if you can encourage the cat to drink as much as possible. Very many cats in their elder years have kidney disease and is a major cause of death, and there is no cure but fortunately these days there is a lot that can be done (as with humans) to give a longer life from diet to subcutaneous and other fluids. Good luck.
Claire Moran ● 2372d