The common causes of hard lumps in breasts during breastfeeding are acute mastitis, milk stasis, and breast hyperplasia. 1. Acute mastitis: It is a suppurative infection of the breast, inflammation of the connective tissue around the milk ducts and the tissue inside the milk ducts, and it is most common in menstruating women. It can be characterized by a lump in the breast with redness, swelling, heat and pain, and in some cases, fever. Acute mastitis is also known as puerperal mastitis. 2. Breast milk stagnation: Breast milk stagnation can also be seen in breast milk stagnation disorder. The milk is not discharged properly from one gland, and the milk accumulates in the breast, which is often misdiagnosed as a breast tumor, and is often seen as a lump in the breast. 3. Breast hyperplasia: For patients with breast hyperplasia, due to changes in hormone levels in the body, breast lumps may occur during breastfeeding and may be accompanied by cyclic breast pain. In addition, breast trauma, breast cancer, breast fibroadenoma and other reasons may also appear breast lumps during breastfeeding, if any breast lumps occur during breastfeeding, it is recommended to go to the hospital in time and follow the doctor’s instructions for treatment.