There are many reasons why there is a hard lump inside the lower side of the breast, such as breast lumps, lipoma, sebaceous cysts, and so on. 1. Breast lumps: a hard lump inside the lower side of the breast may belong to breast lumps, such as breast fibroadenoma, breast lobular hyperplasia, breast cystic hyperplasia, breast cancer and so on. 2. Lipoma: a hard lump inside the lower side of the breast may also belong to lipoma, which is a common benign tumor on the human body surface and can occur in any structure with fatty tissue, especially in the breast. 3. Sebaceous cysts: there are many sebaceous gland tissues distributed in the lower side of the breasts, if the sebaceous glands are blocked, sebaceous cysts can be formed, which will show a hard lump inside the lower side of the breasts. There are many reasons why there is a hard lump inside the lower side of the breast, it is recommended to go to the hospital in time to improve the examination, a clear diagnosis, do not ignore the symptoms, take it lightly, so as not to delay the treatment of the condition.