Mastitis is an inflammation of breast tissue that sometimes involves an infection. It most commonly affects women who are breast-feeding. The inflammation results in breast pain, swelling, warmth and redness. You might also have a fever and chills. Mastitis that isn't adequately treated or that is due to a blocked duct can cause a collection of pus (abscess) to develop in your breast which usually requires surgical drainage. To avoid this complication, consult your doctor as soon as you develop signs or symptoms.