"Inflammation of the breast tissue is known as mastitis. Mastitis may occur in women who are breastfeeding a baby (lactating). When the skin of the nipple (areola) is injured or cracked, which may occur with nursing, bacteria can enter the damaged area and cause infections. In a breastfeeding woman, a hard area commonly thought of as a "clogged milk duct" can form. Sometimes, certain treatments (see below) can prevent the painful, hard area from developing into an actual breast infection. Infections can either be a deep pocket of pus, in which the infection looks like it is growing down into the breast (an abscess), or a wider area of skin redness that spreads out (cellulitis). Body piercing in the nipple area increases the risk of breast infections, and these may be particularly difficult to treat."

Source: https://www.medicinenet.com/breast_lumps_in_women/article.htm#what_are_causes_of_breast_lumps