Breast swelling and pain a few days before menstruation is related to two reasons: first, it is premenstrual syndrome, which is mainly caused by the imbalance of the ratio of estrogen and progesterone in the body during the premenstrual period. Second, there is breast hyperplasia, which is usually accompanied by breast nodules. The production of estrogen and progesterone in the body is decreasing in the days before menstruation, so many women will experience the manifestations of premenstrual syndrome. These symptoms include headaches, back pain, breast pain, irritability, and anxiety, or low work efficiency and memory loss. There is also another condition known as breast enlargement, which can also cause the symptom of breast tenderness in the days leading up to your period. If you want to check if you have breast enlargement, you can have an ultrasound of both breasts to find out.