Yorky problem - need info please

My precious grand dog, Abigail, is 7 1/2 months old. About a week ago my DIL noticed that when she ran (like the wind!) her back right leg wasn't touching the ground. When she examined her, she found the leg was smaller and not as long as the other one. When I felt the legs together, the muscle at the top of the leg that was short, was much smaller than the other leg. It's not noticable because her hair already reaches the ground and the feathers (?) on her legs look like pantaloons.

The vet examined her and found nothing that could have caused this. No pain, even with a thorough testing of range of motion, etc. She is going in Tuesday for Xrays and to have two baby teeth pulled.

My DIL is thinking that it might be a birth defect. She had planned on having one litter of puppies before spaying, but does NOT want to breed her if it is a birth defect.

Has anyone ever had this problem? Any information would be appreciated. This little girl has stolen our hearts. She's going to have a puppy cut before she goes back to the vet to make it easier for the procedures. Sure will miss the "floor-mop" look, but we will put a lock of hair in the baby book.

Thanks for any knowledge you can share.
Pati