A hard lump the size of a soybean in the breast during breastfeeding may be related to breast hyperplasia, milk stagnation, breast inflammation, breast tumor and other conditions. 1. Mammary gland hyperplasia: due to the influence of hormone receptor changes and hormone level changes in the body, the phenomenon of mammary gland hyperplasia may occur during breastfeeding, which is manifested as nodules in the breast, accompanied by cyclic pain in the breast, nipple overflow and other manifestations. 2. Milk stagnation: If the residual milk is not discharged in time when breastfeeding, or if too much milk is secreted, it may gather inside the milk ducts, and in the specific course of time, cysts may be formed, which may be accompanied by pain in some patients. 3. Mammary gland inflammation: If the patient is infected by bacteria, it will lead to the appearance of lumps in the breast, accompanied by obvious redness, swelling and pain and other symptoms, in serious cases, it can form breast abscess, generalized fever and other symptoms. 4. Breast tumor: If the patient has breast fibroid, it will cause lumps in the breast, which is related to the change of estrogen level. If the patient has breast cancer, there will be symptoms such as nipple indentation, lumps in the breast and cellulite-like changes. Breastfeeding breasts have a soybean-sized hard lumps should also be considered breast malignant tumors and breast trauma and other causes, it is recommended that breastfeeding women consult a doctor in a timely manner, due to breastfeeding needs, can not be arbitrarily used medication, should be strictly in accordance with the doctor’s instructions for treatment.