I just found this:
Causes of Premenstrual Breast Swelling and Tenderness
Fluctuating hormone levels account for most premenstrual breast swelling and tenderness. Your hormones rise and fall during a normal menstrual cycle.
The exact timing of the hormonal changes varies for each woman. Estrogen causes the breast ducts to enlarge. Progesterone production causes the milk glands to swell. Both of these events can cause your breasts to feel sore.
According to the National Institutes of Health (NIH), estrogen and progesterone both increase during the second half of the cycle—days 14 to 28 in a “typical” 28-day cycle. Estrogen peaks in the middle of the cycle, while progesterone levels rise during the week before menstruation (NIH, 2010).
Medications that contain estrogen can also cause breast changes such as tenderness and swelling.
Causes of Premenstrual Breast Swelling and Tenderness
Fluctuating hormone levels account for most premenstrual breast swelling and tenderness. Your hormones rise and fall during a normal menstrual cycle.
The exact timing of the hormonal changes varies for each woman. Estrogen causes the breast ducts to enlarge. Progesterone production causes the milk glands to swell. Both of these events can cause your breasts to feel sore.
According to the National Institutes of Health (NIH), estrogen and progesterone both increase during the second half of the cycle—days 14 to 28 in a “typical” 28-day cycle. Estrogen peaks in the middle of the cycle, while progesterone levels rise during the week before menstruation (NIH, 2010).
Medications that contain estrogen can also cause breast changes such as tenderness and swelling.