Border Collies' rear legs do seem different--their hocks seem lower to the ground than other breeds....I attribute this to their quickness in able to change direction in an instant.
But I cant tell if by your quesiton you are referring to her being 'cowhocked' when looking at her from the rear. There's always the chance there's a hip problem rather than leg structure.
Sometimes they can be quite confusing....for instance, Tess was limping on one of her front legs and I thought it was her right leg, as did my brother, and my Dad....and yet at the vets, it was determined to be the left...but he had her walking back and forth a dozen times to try to catch the subtle difference he was looking for. And now, a couple of years later, it is obvious to see he was right in that it was the left leg.