Shoulder pain near the outside of the breast may be caused by breast hyperplasia, muscle strain and other reasons. 1. Breast Hyperplasia: Breast Hyperplasia is caused by proliferation and degenerative changes of breast tissues, followed by endocrine disorders with a close correlation, the more serious cases of breasts will spread to other surrounding tissues and organs, including the peri-shoulder, so there will be peri-shoulder pain close to the outside of the breast position. 2. Muscle strain: lifting heavy objects, exerting too much force, long activity time may occur muscle strain, which will cause chest pectoralis minor muscle, pectoralis major muscle, shoulder supraspinatus muscle, deltoid muscle position is very damaged, resulting in pain. It is recommended to go to the hospital for examination, identify the cause and follow the doctor’s instructions to receive treatment.