My senior cat stopped grooming herself and I don't know if it's a behavior problem or something medical.

At 16, I would push harder for a diagnosis before treating this as a behavior problem. I understand the vet has not found anything specific, but reduced or absent self-grooming in a senior cat is almost always physical before it is cognitive or emotional. The most common reasons I see are pain, particularly arthritis in the spine or hips that makes twisting to groom uncomfortable or impossible, dental pain that discourages any mouth use, or thyroid and kidney changes that affect energy and motivation even when they are not yet at crisis levels.

A cat who was meticulous about her coat for 15 years and has stopped is not depressed in the way we might use that word. She is most likely uncomfortable. Matting develops quickly when cats stop grooming, and it can become painful in itself, which creates a cycle.

I would go back to the vet and ask specifically about a pain assessment and about whether a trial of pain management might be appropriate even without a confirmed diagnosis. Sometimes the response to that trial is itself diagnostic. Cognitive decline is possible at her age and worth discussing, but I would want physical pain to be more thoroughly ruled out first. If you want to talk through her full picture, what she eats, how she moves, what her days look like, the Work With Me assessment gives me enough detail to help you think through the right questions to bring back to your vet.