Hi, I am Dr. Peter and would like to help.
1- Is she currently on any medications?
2- Any pre existing medical conditions?
3- Can you describe what is happening to her back leg?
I will recommend you speak to your veterinarian about putting her on arthritic medication like Rimadyl. In the meantime, you can give her buffered aspirin 5-10mg per pound body weight every 12 hours with some food. So, for a 13 lbs dog you can give one 81mg baby aspirin every 12 hours with some food. Try not to keep her on aspirin for more than a week as it can cause GI irritation. In addition, supplement with Glucosamine 25mg per pound a day long term. Glucosamine helps by promoting joint fluid; it takes about 4 weeks to see significant improvement.
If you have additional questions I will be glad to answer them.