You may be describing an injury to the long thoracic nerve that controls the serratus anterior (SA) muscle. When there is weakness in the SA the shoulder blade or scapula can’t rotate and slide like it should when the arm is lifted up. The result is called scapular winging.
Mastectomy is one possible cause for this problem. During the surgery for the mastectomy, the nerve can get stretched from the position of the arm. There are many other possible reasons such as a viral illness, working with the arms overhead, or lifting a heavy weight.
The best way to know for sure what’s going on is to have some testing done. Electromyography or EMG studies can find out for sure which muscles are involved. Nerve conduction velocity (NCV) tests can be done to see if the nerve is damaged. Talk to your doctor about these symptoms and see what he or she suggests.