Have Cat Questions? Ask a Cat Vet Online.
At this age, renal failure is always the biggest concern. Kidney failure is common.
If this is the case, your vet can diagnose it with some simple bloodwork and a urinalysis.
Many cats get better just with a diet change to a prescription renal diet. IV fluids are also used to flush the kidneys. Drug therapy is also used.
Keep in mind that renal failure can cause blindness so this might be why you are seeing the stumbling and calm.
No matter what, you don't want to take any chances at this age so you will want to see the vet.
Let me know if you still have questions.
As long as she is eating and drinking well, you should be able to wait. If you feel uncomfortable with it at any time, find an after hours vet.