Breast swelling on one side and softness on the other, that is, swelling on one side of the breast, may be caused by breast hyperplasia, if breastfeeding women are considered to be milk stagnation, but may also be the presence of breast cysts. 1. Mammal hyperplasia: Mammal hyperplasia is a relatively common benign breast disease in clinic, which is generally caused by the drastic changes in hormone levels in the patient’s body. The main symptom of the disease is pain in the breast area, and there will be breast nodules or hard lumps. 2. Milk stagnation: Milk stagnation usually occurs in breastfeeding women. If the milk is not discharged in time after breastfeeding, it will cause milk stagnation, which will lead to swelling and pain in the breast area. 3. Breast cysts: Breast cysts are round or oval cystic masses in the patient’s breasts, which may occur in both breasts or only in one side of the breast. Breasts with breast cysts are usually swollen, and a swollen breast with a soft side indicates that there may be a cyst in one side of the breast. In addition, mastitis, breast trauma, breast tumors may also appear similar clinical manifestations, in the presence of swelling in the breast area, should consult a doctor in a timely manner, to clarify the cause of the disease, under the guidance of the doctor for treatment.