Hey I'm not sure whats happening here either all I can offer is that my girls friends cat had the very same thing happen..came home aft neutering and attacked his tail non-stop after, she kept phoning the vet and he kept saying it was anxiety(could be)...but interestingly enough AFTER she bought him a flea collar it stopped. she again phoned the vet office and told them this and said he must have contacted fleas at their clinic, of course this was denied..seems a coincidence to me tho.
good luck...
also I've read in a vet forum that if it is fleas and the collar wearing doesn't help(neck along way from tail) that a shot of cortisone will kill all
I'm not sure what could be happening, but I would call the vet's office and tell them what's happening and ask them what they think. Good luck!