A little pain on the side of nipple may be caused by localized friction, mastitis, breast hyperplasia and breast cancer. 1. Localized friction: nipple nerves are densely distributed and the skin is relatively thin, when running exercise is carried out, it may rub against the clothes, thus leading to pain, this situation does not need special treatment. 2. Mastitis: If the painful area has redness and heat, it is likely to have mastitis, note that it is due to bacterial invasion, and if not treated in time, an abscess may appear. 3. Breast hyperplasia: pain on the side of the nipple is a more common symptom of breast hyperplasia, most of which is related to endocrine changes, and it is more obvious before and after menstruation, accompanied by lumps in the breast. 4. Breast cancer: Breast cancer is a kind of malignant tumor, when the lesion of breast cancer involves the nipple to the root, it will cause pain in the nipple, but this kind of pain will not appear frequently. At the same time, breast duct dilatation, breast duct papilloma, cracked nipple and so on will appear pain on the side of the nipple, need to be carefully identified. There are many other causes of a little pain on the side of the nipple, which should be examined promptly and treated under the guidance and advice of a doctor.