Dog shaking and biting her tail

Question
I have a 1 year old Shitz Su/ Yorkie mix that just started shaking and trying to bite her tail.  It's almost like she is in pain and lunges at her tail. Occasionally, she shakes as if she's cold.  When I bring her out for a walk, she shakes her body like she's trying to get someting off it (a lot like when she's wet) and sits down like it's painful to walk.  I brought her to the vet and her tests were fine.  What do you think this could be?  I'm very concerned.

Regards,
Vinnie

Answer
Things that I would consider are: pain, fear, small dog shaker syndrome, sensitivity to cold, anxiety.  If she lunges at her tail, I might also think about disc problems, anal sacs full, irritation from grooming (brush burn, clipper burn).  A lot of small dogs shake for no apparent reason (although I'm sure there is one).  Maybe you can video the dog and show the video to your vet for further analysis and to rule out any painful condition.  I know at least one dog owner who didn't get a diagnosis until the dog was x-rayed.  Good luck.