Mastitis

"Mastitis is a condition that causes a woman's breast to become swollen, tender, reddened and painful. Mastitis is most often caused by a bacteria known as Staphylococcus aureus. This condition commonly affects women who breastfeed their babies, and it can make nursing difficult and uncomfortable. Mastitis may be present with infection, so women who suspect they may be suffering from this condition are advised to consult a doctor who can determine whether antibiotics are necessary."

- From "Common breastfeeding concerns: what is mastitis?"

Other sources:

"Common breastfeeding problems: all about mastitis" - Breastfeeding.com

"Mastitis" - AskDrSears.com

"Mastitis" - MayoClinic.com

See also:

Thrush