The cause of fibrocystic changes is related to the way breast tissue responds to monthly changes in the levels of estrogen and progesterone, female hormones produced by the ovaries during a woman's reproductive years. Each month during one's menstrual cycle, the breast tissue alternately swells and returns to normal. Hormonal stimulation of the breast tissue causes the milk glands and ducts to enlarge, and the breasts to retain water. The breasts frequently feel swollen, painful, tender, and lumpy at this time. After menstruation, swelling in the breast usually decreases, and the breasts feel less tender and lumpy. That's why the best time to examine the breast is 7-10 days after the start of the menstrual period, when the breast tissue is at its most normal state.