The main causes include the following: 1) inflammation around the nipple, which is more common because the nipple skin is thin and easily broken, and bacteria take advantage of the situation to cause nippitis, which can be red, swollen and painful, with some exudation in severe cases. 2) It may be due to nipple chalazion or nipple eczema, due to the appearance of eczema-like changes, there will be redness and swelling at the skin with significant pain. In conclusion, it is important to go to a regular hospital for a detailed examination and take different treatment plans according to the cause.