The common cause of hard lumps and pain during breastfeeding is mastitis, but the possibility of inflammatory breast cancer cannot be ruled out, so it is recommended to consult a doctor in time for examination and treatment. 1. Mastitis: Mastitis is more likely to be the cause of hardness and pain during breastfeeding. This is mainly due to the fact that the milk is not discharged in time during the breastfeeding period, which leads to the accumulation of too much milk, resulting in localized swelling and pain, hardness and hardness. Or due to the bacteria in the nipple retrograde upward infection of the breast ducts, the internal infection of the breast to form a lump, the surface of the red, swollen, hot, painful, accompanied by a generalized fever. 2. Inflammatory Breast Cancer: If there is a hard lump and pain in the breast during breastfeeding, the possibility of breast cancer cannot be ruled out. Inflammatory breast cancer has a very low incidence rate and is a special type of breast cancer, which mainly manifests itself as redness, swelling and pain in the whole breast, similar to the performance of breastfeeding mastitis, but the symptoms cannot be relieved through anti-inflammatory treatment. If you have hard lumps and pain in your breasts during breastfeeding, you should consult a doctor in time and have a molybdenum target examination to diagnose the nature of the lumps, so as to choose the appropriate treatment.