I too had a modified radical mastectomy, with subsequent chemo & radiation. I have lots of scar tissue - lymphodema in the arm & chest. did not have reconstruction. I have extensive cramping in the chest, abdomine, groin, legs, neck. I have talked with doctors, including neurologist, and they cannot find any reason for the cramps - have to live with them. They are getting worse over the years! Muscle relaxers do seem to help some - at least keep them from waking me up at night. Things that I have recently considered doing - when I get over the current pneumonia, is accupuncture & ballet exercises. You might want to try them....
I really thing you should consult a doc about this...it doesn't sound normal. Muscle spasms can be painful, but this seems excessive. Good luck