What is wrong with my cats tail?
I haven’t the foggiest idea. I’ve never heard of this problem before without some sort of injury to the tail. It could be that he hurt his tail and you didn’t know. Or he could have a nerve problem (such as from diabetes – but usually diabetic neuropathies in cats affect their hind legs). Or it could be a tumor. Or he’s just choosing not to move it and it’ll be fine tomorrow. If I were his vet, I’d probably want X-rays of the tail and blood work to check for metabolic problems like diabetes to try and find out what is going on.