Has she had any injuries, IntrepidS? She definitely needs to go to the vet, I'm afraid. Dogs can have bone injuries, joint injuries, tendon injuries, hip dysplasia, breaks, sprains--just like people. They also can have nerve injuries that cause contracture or paralysis. And they can have little strokes or spinal cord injuries that cause problems with limbs, too. This is why she needs to go to the doc. They will be able to tell a lot just by looking at her and watching her walk. They also do a test where they set the dog down on a counter or tabletop and watch how she uses the back leg when it encounters that surface.

My dog, Daisy, had a dislocated knee once that caused her not to be able to walk on her back left leg. My vet said that was a very common injury in small dogs. We had that one surgically fixed, which wasn't cheap, but it turns out that she has the same problem with her other knee, and it just sometimes pops out of place but then pops right back in. Good luck!