Problem with tail

I have a seven year old shih tzu that has begun having problems with his tail. He does not hold it up and it just drags behind him. The first time this happened (about a year ago) we thought he had experienced some sort of trauma, because he was experiencing pain and yelping when he would try to poop. Several x-rays did not reveal any trauma. He has had two other episodes since then and now he is on prednizone to try and fix it. Has anyone else experienced this kind of problem? My vet thinks it may be an alergy issue instead of trauma since it has happened three times now. His is showing signs of swellling at the base of his tail and redness in the same area.

Any help would be appreciated.

Sandi