I understand you are being treated for breast cancer, since you mentioned you are taking Tamoxifen.  My question is … did you have radical mastectomy? Simple mastectomy? Quadrant resection? Axillary lymph node dissection?  Clarification as to the extent of the surgery would be helpful in response. 

            Losing mobility in your left arm concerns me, especially with associated pain.  Indeed the direct radiation can lead to fibrosis, if your are not consistently doing range of motion exercises.   I would recommend relaying this information to your physician and requesting basic f/u assessment and, if appropriate, referral for physical therapy support to help with your range of motion and movement.