The following aspects are considered to be the cause of the pain under the breast: Firstly, it is considered to be caused by elevated estrogen, which is a problem that occurs every month in women, rising and falling with the menstrual cycle, and when it rises, glandular hyperplasia will occur under the stimulating effect of the breast, and after the end of menstruation, the glandular body will return to normal. Secondly, it is considered to be benign or malignant tumor. As the tumor keeps increasing, benign or malignant tumor will produce pressure on local tissues and nerves, especially malignant tumor will also release related inflammatory factors, so it will produce pressure pain. Thirdly, it is considered to be suffering from inflammatory infection, i.e. mastitis, which is caused by excessive milk stagnation without timely suction, or due to retrograde upward infection of bacteria.