My Shih Tzu seems to have a problem with his tail. He's never been aggressive but when you even look or try to touch his tail he tries to bite you. I have checked it up and down and even combed it out and can't find any bumps or sores. He's a very affectionate dog and not a biter. Thinking of having a vet check him even though I can't find a thing. Could it be his glands?