Pain in the outer side of the armpit of the breast may be caused by mastitis, premenstrual distension, breast bone arthropathy and other reasons. 1. Mastitis: mostly caused by milk stagnation, mainly manifested as hard lumps in the breast, breast heat and pain, axillary pain in the outer side of the breast, serious cases may be extruded pus secretion, accompanied by swelling and pain, and some will also have a rise in body temperature. 2. Premenstrual swelling and pain: some patients will experience breast discomfort before the first menstruation, such as pain in the outer side of the breast in the armpit, or even granular lumps, which is actually caused by the increase of estrogen level in the body. 3. Mastoid arthropathy: Mastoid arthropathy is very much related to estrogen and phytoneurotic metabolic abnormality. The basic principle of the disease is the same as that of primary menstrual pain, which is mainly reflected in the pain on both sides of the breasts, and may even radiate to other locations around the breasts. It is also important to consider breast hyperplasia, breast tumors, as well as trauma to the breast and extra-mammary diseases. If the patient has the symptom of pain on the outer side of the breast in the axilla, he should first go to the hospital for a surgical examination, through which the cause of the pain will be determined, and then be given the appropriate treatment.