This is usually caused by either a urinary tract infection or crystals in the urine, both of which can be pretty serious in a cat this age.
I would go ahead and see the vet tomorrow for a urine culture to find out what you're dealing with.
Until then, I would try to increase her hydration by offering some plain pedialyte in replace of her water.
Then see your vet tomorrow.
Let me know if you still have questions.