Mastitis is an infection of the breast tissue, usually, but not always in women after nipple piercing and other procedures.
Nipple piercing-related factors can lead to serious problems with a woman's breasts. Infected milk ducts are a possible risk when dealing with nipple trauma. This condition is similar to a problem breastfeeding women can experience.
Normal signs of an infected piercing may not even be visible. There might be a lump (warm to the touch, possibly discolored) that forms somewhere away from the nipple on the breast.
Antibiotics can often treat them, or, in slightly more serious cases, lancing (like you'd do for a boil) can be done in the doctor's office ( I&D, incision and drainage). Untreated though, it can easily lead to surgical removal of the infected milk ducts or the entire breast. Worst case scenario is death.
In any case, when it occurs it is essential that this type of infection be treated immediately to avoid more serious problems.