Breast

"The breast is the upper ventral region of the torso of a primate, in left and right sides, which in a female contains the mammary gland that secretes milk used to feed infants. Both men and women develop breasts from the same embryological tissues. However, at puberty, female sex hormones, mainly estrogen, promote breast development, which does not occur in men, due to the higher amount of testosterone. As a result, women's breasts become far more prominent than those of men.
During pregnancy, the breast is responsive to a complex interplay of hormones that cause tissue development and enlargement in order to produce milk. Three such hormones are estrogen, progesterone and prolactin, which cause glandular tissue in the breast and the uterus to change during the menstrual cycle. Each breast contains 15–20 lobes. The subcutaneous adipose tissue covering the lobes gives the breast its size and shape. Each lobe is composed of many lobules, at the end of which are sacs where milk is produced in response to hormonal signals."

Mastitis

Mastitis is an infection of the breast tissue that results in breast pain, swelling, warmth and redness of the breast. If you have mastitis, you might also experience 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.
In most cases, lactation mastitis occurs within the first three months after giving birth (postpartum), but it can happen later during breast-feeding. The condition can leave you feeling exhausted and run-down, making it difficult to care for your baby.