Mastitis is an infection of the breast tissue that results in breast pain, swelling, warmth and redness. Patient also might have fever and chills. Mastitis most commonly affects women who are breast-feeding (lactation mastitis), although sometimes this condition can occur in women who aren't breast-feeding. Sometimes mastitis leads a mother to wean her baby before she intends to, but continuing to breast-feed, even while taking an antibiotic for the mastitis, is better for you and your baby.