I have had a mastectomy and lymph nodes removed and have had yhis problem and I know what causes this.
Lymph nodes create lumph fluid and once they are removed the fluid is supposed to be absorbed into your body on that side. The fluid that is not absorbed will crystalize around muscles and tendons and therefore the more it hardens the more it aches and hurts. Also if not tended to it can cause lymphodema if a blockage of fluid occurs. Lymphodema as of date canot be cured only managed. Your arm will swellup and not deflate.
I am a 14 year breast cancer survivor and what I have learned is that crystalization needs to break away from the muscles and tendons.
The best way I have to do this is with stretching my arm over my head and pulling in an upward motion across the top of my head and leaning over toward the pull. This stretches the muscles and tendons and cracks off the lymph fluid crystals. At first it may be uncomfortable but then you'll get the 'AHHHHH" goof feeling. I do this exercise morning, noon and night daily and have never had this problem again.
Hope this helps!
Vicki325