Question about cat's tail

I have a 16-year-old indoor cat that cannot hold up the last 1/3rd of her tail. This occurred after a routine dental. What could have caused this? Injury to back during recovery, or small spinal cord stroke? Her tail is not painful at all. She does seem to have arthritis in her hips and make have arthritic changes in her spine as well.

Only your vet could tell for sure, but all are likely possibilities. Have you discussed this with the vet, yet?

Yes and he has no idea.