One side of the breast pain, the other side of the breast does not hurt the following reasons: First, mastitis, due to the postpartum women need to breastfeed babies, if not in time to breastfeed or not in time to use a breast pump to suck out the breast, too much milk will be stagnant in the mammary glands, which may result in mastitis, mastitis is mainly red, swollen, hot, pain as the main manifestation; if nipple breast milk breeding bacteria and not cleaned up in time, may If the nipple milk breeds bacteria and is not cleaned in time, the bacteria may go backward and upward and cause mastitis; secondly, breast nodules and breast cysts, which are mainly caused by changes in the level of oestrogen in the body, will stimulate the nerves in the breast and may cause pain; thirdly, trauma leads to soft tissue contusion, which will cause pain; fourthly, consider having a tumor mass.