If your breasts are soft during breastfeeding but one side hurts when you press on it, it may be caused by breast milk stagnation, mastitis or breast hyperplasia. 1. Breast milk stagnation: During the breastfeeding period, the breasts will produce a lot of milk, and sometimes the milk will be stagnant in the milk ducts, and the breasts will be enlarged and soft in the early stage, and it will be painful when pressed. 2. Mastitis: If the milk is stagnant for a long time, it may lead to mastitis, which causes redness and swelling of the local tissues, and then the breasts will be soft and painful when pressed. 3. Mammary gland hyperplasia: due to the change of estrogen concentration during the breastfeeding period, it will often cause different degrees of mammary gland hyperplasia, and some women will also have the symptom of pain when pressing on one side of the chest, which will slowly improve after the breastfeeding period. If your breasts are soft and one side hurts when pressed, you should also consider other causes such as breast trauma and breast tumors. During the breastfeeding period, you need to actively observe the changes in your breasts, breastfeed regularly and pay attention to a light diet, and if your breasts are soft and one side hurts when pressed, it is recommended that you consult a doctor for examination in a timely manner.