You can thank hormones for that. A spike in estrogen just before ovulation causes the breast ducts to enlarge, while a peak in progesterone during the week before your period stimulates the growth of the milk glands. The resulting breast soreness, aching or swelling often goes hand in hand with PMS (ugh!), intensifying during the days leading up to your period.
While most women have tenderness in the upper, outer portions of both breasts, you can feel it all over and even in the underarm area (where some women have small amounts of breast tissue).
To ease the pain, try wearing a comfy, supportive bra, taking an over-the-counter pain reliever and keeping up with regular exercise. You may find it helpful to limit caffeine, salt, and fatty foods in the weeks before your period; you can also ask your doctor about trying a daily regimen of vitamin B6, vitamin E, and evening primrose oil supplements. After your period begins, the tenderness typically starts to go away. However, if it continues daily for a few weeks, worsens over time, or seems to be in one particular area of your breast, see your doctor.