Breasts that are soft but painful during breastfeeding can be caused by prolonged breastfeeding, poorly opened milk ducts and viral infections. 1. Too long breastfeeding time: During breastfeeding, if the breastfeeding time is too long, the milk in the breast is sucked away by the baby, and after the breast is emptied, it will lead to soft breast, and it will stimulate the pituitary gland as well as the mammary glands, leading to the secretion of a large amount of lactotropin, which will cause spasm of the mammary ducts and lead to pain. 2. Unobstructed milk ducts: It is possible that the milk ducts are not completely blocked, and there is no obvious change in the texture of the breasts, and you cannot touch the lumpy tissues with hardened texture. It is recommended to massage the breasts or apply hot towels to unblock the milk ducts, which will relieve the pain caused by the accumulation of milk. 3. Viral infection: Breasts are soft but painful during breastfeeding may be the result of a viral infection of the tissues in the mammary glands, and mild symptoms of the infection can be painful and need to be examined. The above reasons may be the cause of breastfeeding breasts soft but painful, find out what kind of causes, if it is caused by disease should go to the hospital in time for examination.