M.L.
I have worked with animals hospitals and vet clinics and acting out can be a common behavior. Older cats (she would be considered a senior due to her age) can become aggressive or terratorial if they have a sickness, or pain, perhaps from arthritis, diabetes or thyroid issues. Or, it could be as simple as the heat being out. A senior cat bloodwork lab may provide some insight.
Keep track of odd behaviors, if she is having trouble going to the bathroom, witholding pee, or peeing very frequently, not eating/drinking or over eating and being constantly thirsty... it could help you find a pattern to see what is going on.