There are many reasons for a small hard lump inside the nipple, and the specific analysis is as follows: 1. Mastitis. If the breast duct deformity or nipple deformity, resulting in milk stagnation in the body appears mastitis. Patients with mastitis will have sore, swollen nipples and be able to feel small hard lumps inside. The milk needs to be emptied and erythromycin ointment applied, which will heal in a few days. If there is pus production, puncture to drain the pus or surgical incision to drain the pus is required. 2. Dilated milk ducts. If the lump is a tumor, surgery can be performed, followed by a histopathological examination for benignity and malignancy, and then a treatment plan can be decided. 3. Lymphadenitis. Lymph nodes in the breast are stimulated by inflammation, resulting in swollen and painful lymph nodes, which require anti-inflammatory treatment by infusion. If the cancer is metastatic leading to swollen lymph nodes, high vigilance is needed.