Even though the doc might not know the exact cause, he/she should have told her what she can do to help decrease the pain. She ought to call the doc's office & ask what she should use for the pain. Pain is often treated with heat or ice, but which one to use depends on what is causing the pain. Heat can make some injuries worse, can increase swelling, if there is any. All I can really say is that she should stay off that leg & rest it as much as possible. If there is any swelling in her leg, she should keep it elevated whenever she can, and if her pain worsens, becomes severe, or her leg becomes very swollen, if the skin turns red, there is any muscle weakness, etc... if anything changes drastically from the way it looked/felt was when she went to the doctor, she should to go to the ER.
I found some stuff online called Soothanol X2. It is suppose to be AMAZING. It is kind of expensive but only 2 drops is all you should need. Tell your mom about it and see if she thinks it is worth it.
Have drs done a Doppler study to rule out blood clot . Which if this blood clot travels can cause a heart atrack or stroke. not to scare you but please get one to rule out clots. This is symptom of in my practice as a nurse .
I would for sure call that Doctor and ask for something for the pain. Even a couple of ibuprofen can reduce any inflammation. I had the same type of problem and it was a pinched nerve. Started in my upper leg then my knee, then some in my lower back and then the other leg started hurting too. Doc gave me some pain pills and some 800mg Ibuprofen. I finally went to a Chiropractor who also did Acupressure. Within a couple of weeks, I've felt a LOT better. Something that you might mention to your Doctor! The Chiro and Acupressure really works and I don't touch the pain pills anymore. Ice is a good relief! Call that Doctor and ask if it could be a pinched nerve or Sciatica.