Mastitis is a painful condition of the breast which becomes red, hot and sore (inflamed). It is usually caused by a build-up of milk within the breast (a blocked duct or engorgement). Sometimes it can be caused by germs (bacteria) that get into the milk ducts of the breast. This is often through a crack or sore in the nipple.
This is a companion discussion topic for the original article at https://patient.info/pregnancy/feeding-your-baby/mastitis-and-breast-abscess