Breast pain on one side of women is caused by mastopathy, mastitis, breast cancer and other diseases. The specific cause of mastopathy is not clear. Firstly, it is considered to be related to endocrine disorders and bad habits, such as eating high-protein food, smoking, drinking and taking birth control pills frequently, the body will take in a large amount of estrogen, which causes mastopathy. Secondly, changes in hormone levels in the body during pregnancy and pre-menstruation lead to pain in the area with lumps. Once again, it is considered to be caused by mastitis. After the baby is born and breastfeeding for the first time, the nipple area is easily damaged due to the delicate nipple area and the baby’s sucking, which can easily cause nipple infection when Staphylococcus aureus invades, and local lumps are formed when the breast ducts are blocked and milk cannot be discharged, causing breast pain. Finally, genetic, hormonal, environmental and other factors lead to multiple genetic changes in the body, thus triggering the development of breast cancer, which is a malignant tumor and causes breast pain when the lump grows continuously.