Mastitis is an inflammation of the breast tissue, most often caused by an infection. Mastitis is most common in lactating women. Symptoms include a hard, firm area in the breast that is usually painful. Other symptoms include warmth or redness of the overlying skin, pain in the nipple area, and swelling. Pain while breastfeeding, fever, chills, and generally feeling unwell (malaise) can be other symptoms of mastitis. Sometimes, blood or pus can be seen in breast milk when mastitis is present.