This sounds like vestibular disease a problem that can be in the brain but most often is in the inner ear which is the balance center.
Sometimes antibiotics or steroids may help to clear this faster but it can also clear on its own.
You can read about this here
http://www.cathealth.com/vestibular.htm
Another possibility in a senior cat is a problem with arthritis making the joints sore and leading to instability due to muscle loss from lack of use.
I would plan a vet visit to have your senior cat checked out to determine if treatment is needed and if so of what type.
Hope this helps you!
Cat Health, Behavior, Care Expert
30+ years cat owner, rescue, breeding, study of behavior & health care
Thank you for your input. I will certainly continue to monitor her and make an appt if her symptoms become more pronounced.
As a side note, I don't think it's inner ear, as she does not have a head tilt - honestly, I am thinking that it is either a brain lesion (from a fall or misjudged leap) or a tumor. And, one will heal, and the other will progress. I know what my money is on...
Thank you for your input.
You are welcome and thanks for the above accept.
Other things to watch for - more drinking and urination (kidneys or diabetes), dropping off eating (liver or kidney issues), rear leg coordination issues (can go with a heart problem due to a blood clot reducing rear leg blood flow), excessive hunger and weight loss (thyroid problem or diabetes).
A senior cat check up with blood work can catch things early enough sometimes to treat them.