Hi,
It would be very difficult to find out the cause of pain or to give a confirmed diagnosis without a proper clinical evaluation.
She needs to follow up with her breast specialist.
Has she noticed any swelling in the region?
She could take some pain relief medications and try some warm compresses till she gets an appointment with her doctor.
Let us know if you have any other doubts.
Hope this helps.
Regards.
Sounds like she probably has some lymphedema...swelling of the skin and muscles in the affected arm. She needs to ask her doc to give her a prescription for a sleeve and a gauntlet. Also, if they will, see if she can get in with a physical therapist to help her reduce some of the swelling that has occurred and learn how to manage the swelling with manual lymph drainage.
It just happens to some people...especially those of us who try to resume our activities (because we have to) before our bodies are ready to handle it all.
One thing she can do, is called 'efflurage' a light massage (barely touching the skin) from the fingers to the top of the shoulder and elevate the arm at night. That should help some until you can convince her to ask for some therapy.