Benign breast cyst or fibroadenoma: These are common breast lesions that can present as lumps and are often found behind the nipple. They are usually benign and can be diagnosed with imaging and clinical examination.

Mastitis or breast abscess: Inflammation of the breast tissue, which can cause a lump behind the nipple, especially if it becomes infected and forms an abscess.
Ductal ectasia: A condition where the milk ducts beneath the nipple become dilated, leading to a lump or thickening behind the nipple.
Intraductal papilloma: A small, benign tumor that grows in the milk ducts of the breast, often causing a lump behind the nipple.

Breast cancer (e.g., ductal carcinoma in situ or invasive ductal carcinoma): Although less common, breast cancer can present as a lump behind the nipple. Early detection is crucial for effective treatment.
Paget's disease of the breast: A rare form of breast cancer that affects the skin of the nipple and, sometimes, the areola. It can cause a lump or thickening behind the nipple.

Hamartoma: A rare, benign tumor of the breast that can present as a lump.
Phyllodes tumor: A rare tumor of the breast that can be benign, borderline, or malignant, presenting as a lump.
Fat necrosis: A condition where the fatty tissue of the breast becomes damaged, leading to the formation of a lump, which can be mistaken for a tumor.
Sarcoidosis of the breast: A rare condition where granulomatous inflammation affects the breast tissue, potentially causing lumps.