When a German Shepherd limps, it is likely a sign that there is something more going on than a simple injury.
Visitor: My German Shepherd Limps
My one-year-old female German Shepherd occasionally limps. She plays hard, digs holes and loves to run. Now she has begun limping badly on her back leg and holds it up when she stands. We have checked her pads and joint movement, but she doesn't complain.
My hunch is that your dog may have some degree of dysplasia. What was once a minor pain is now becoming more of an issue as your dog grows and gains more weight.
When you checked your dog's joint movement she probably wasn't putting any weight on it, was she? While she may still be able to tolerate the actual movement of the joint, her own weight will put pressure on the joint and cause the bones to grate painfully against each other.
Veterinarians use X-rays to check for dysplasia, so I recommend that you make an appointment for your bitch right away. This degenerative disease is rather common in your breed, but your vet may determine the cause of your dog's limping is something as simple as a stress fracture. Either way, you need to find out.
Thank you for your question, and I hope your dog's condition is something that can be resolved.