Hard lumps in breasts during breastfeeding but no pain, if it is mastitis, milk stagnation, usually not serious, if it is a malignant tumor, relatively more serious. Hard lumps in breasts during lactation are mostly related to milk stagnation. If you don’t insist on letting your child suckle, it will lead to blockage of milk ducts, which is easy to form lumps and will induce acute mastitis. It can be controlled by timely treatment and the milk ducts can be unblocked. However, if there is a painless hard lump during the breastfeeding period, we should be highly alert to the fact that it is caused by breast cancer, which is a malignant tumor. It is a malignant tumor, which is more serious and cannot be completely cured at present. If it is in the middle or late stage, the cancer cells may spread and cause metastatic cancer, which may even threaten your life. Besides, there may also be cystic fibroma of the breast, which may also manifest as painless lumps. If you feel a hard but painless lump in your breast during breastfeeding, it is recommended to go to the hospital as soon as possible for examination, such as breast ultrasound, mammography and tumor marker screening, to clarify the cause and then go for standardized treatment.