Painful hard lumps in breasts during breastfeeding may be caused by milk stagnation, mastitis, breast nodules and so on. 1. Breast milk stagnation: It may be caused by improper feeding or the milk can not be secreted smoothly, resulting in breast milk stagnation, which will form hard lumps in the breasts, accompanied by a feeling of pain. 2. Mastitis: Inflammation caused by bacterial infection of the milk ducts, resulting in milk not being able to be discharged from the body, resulting in a hard lump in the breast, accompanied by pain. 3. Breast nodules: The cause of breast nodules is unclear and is related to a variety of genetic and environmental factors. Currently, the possible causes of breast nodules include endocrine hormone levels, genetic mutations and environmental influences, etc. The formation of nodules will be accompanied by pain. Painful hard lumps in breasts during breastfeeding may also be seen in diseases such as breast tumors. If symptoms of painful hard lumps in breasts during breastfeeding persist without relief, prompt medical treatment is needed.