Nipple inversion on one side of the breast may be caused by breast cancer, mastitis or congenital dysplasia. 1. Breast cancer: It is a cancerous transformation of breast epithelial cells that occurs under the action of multiple pathogenic factors or cancer-causing factors. The first symptom is a painless lump in the breast, which may be accompanied by cellulite changes in the breast skin and nipple discharge, and when the cancerous tissue invades the nipple or areola, it may pull the nipple to make it concave. 2. Mastitis: Inflammation of the mammary gland caused by microbial infection or other reasons. It can be categorized into breastfeeding mastitis and non-breastfeeding mastitis. The former often manifests as redness, swelling, swelling and pain in the breast, while the latter is mainly manifested by breast lumps, painful breast abscesses, and nipple indentation. 3. Dysplasia: If the muscles of the nipple or areola are underdeveloped and the mammary ducts are short, the nipple will be sunken due to insufficient support. It should be noted that if nipple depression appears suddenly within a short period of time, no matter with or without other symptoms, you should go to the hospital for examination in time.