Pain in the shoulder, arm, and outer breast may be related to breast hyperplasia, mastitis, breast tumor, and other causes. 1. Mammary hyperplasia: due to the change of estrogen level before menstruation, it can lead to cystic hyperplasia of the breast and breast pain, which may involve the shoulder and arm. 2. Mastitis: it often manifests as localized pain in one breast, and in severe cases, it can cause swelling and pain in the lymph nodes in the armpit, and may also involve shoulder and arm pain. 3. Breast tumor: it may cause pain in the breast and may involve the shoulder and arm. In addition, breast trauma, cardiothoracic diseases may also appear similar clinical manifestations, it is recommended that people with shoulder, arm and lateral breast pain go to the hospital in time to clarify the cause of the disease and standardize the treatment under the guidance of doctors.