Nipple swelling and pain may be caused by local irritation, high estrogen level, acute mastitis, cystic breast hyperplasia, breast cancer and so on. 1. Local stimulation: wearing too tight and breathable underwear for a long time, resulting in breast compression and stimulation, moist and sweaty skin in the nipple areola area, frequent use of irritating soap and other cleaning products to wash the nipple areola, and nipple injuries resulting in hematomas and other physiological factors may lead to nipple swelling and pain. 2. Excessive estrogen level: Estrogen secretion increases during puberty and secondary sexual characteristics begin to develop, which may lead to excessive development of mammary ducts and nipple swelling due to high estrogen level. In addition, before and after ovulation and during pregnancy, estrogen will also rise and cause nipple swelling and pain. 3. Acute mastitis: commonly found in breastfeeding women, due to excessive milk secretion blocking the mammary gland ducts, improper breastfeeding leading to nipple injury, bacteria and other microorganisms through the nipple retrograde infection of the mammary gland, causing inflammatory lesions of the mammary gland, which can cause nipple swelling and pain. 4. Breast cystic hyperplasia: after the expansion of breast lobules, ducts and other tissues to form cysts, there will be nipple swelling and pain, breast lumps and other manifestations. 5. Breast cancer: special types of breast cancer, such as nipple eczema-like cancer, manifested as unilateral nipple, areola and its surrounding swelling and pain, itchy skin, red patchy eczema-like appearance, the surface of the oozing crust or keratinization and flaking, and ulcers can be formed in serious cases. Nipple swelling and pain may also be caused by nipple trauma, cracked nipples, nipple inflammation, etc. It is recommended to consult a doctor as soon as possible for a clear diagnosis.