Your breasts contain benign lumps or cysts. Hormonal changes stimulate the cysts, which may become larger and more painful before your period. "It doesn't mean you're more likely to develop breast cancer," says Therese Bevers, M.D., medical director of clinical cancer prevention at the University of Texas M. D. Anderson Cancer Center in Houston. Stick with your regular schedule of breast checkups-a monthly self-exam and an annual clinical exam, plus a yearly mammogram if you're over 40. "If you find a new lump, tell your doctor, who will likely investigate it with an ultrasound or biopsy to confirm that it's benign," advises Dr. Bevers.