A common cause of a breast infection is normal bacteria from your skin entering the milk ducts through a crack on, or other damage to, your nipple.  Most likely this crack is from a shallow latch. Sometimes a breast infection follows a plugged duct and can be caused by the same things: irregular feeding patterns or missed feedings, tight clothing (bras, bathing suits, etc.), pressure on the breast from a baby carrier, heavy purse or diaper bag, sleeping on one’s stomach and unusual stress or fatigue.