There are many reasons why red bumps on nipples hurt during breastfeeding, such as mastitis, skin allergy, eczema-like cancer of the nipple and so on. 1. Mastitis: Mastitis may be induced by milk stagnation, which may lead to the phenomenon of red bumps on nipples that hurt during breastfeeding, accompanied by obvious redness, swelling and hard lumps in the breasts. 2. Skin allergy: nipple contact with allergens may induce localized skin allergies, which can lead to the phenomenon of red bumps on breastfeeding nipples is very painful, and may also be accompanied by eczema. 3. Eczema-like nipple cancer: Eczema-like nipple cancer can cause painful red bumps on breastfeeding nipples, accompanied by itching, erosion, pain, rash and other phenomena, which need to be highly vigilant. Meanwhile, papillitis, eczema and other diseases will also appear similar symptoms. In short, there are many reasons why red bumps on breastfeeding nipples are painful, and it is recommended that patients go to the hospital in time to improve the examination, and take standardized treatment under the guidance of the doctor, so that they can not ignore the symptoms and take them lightly.