Any breast pain you may feel before your period is caused by the changes in your hormones. These hormones cause fluid to build up in your breasts, and this can make them sore and/or feel heavier. The good news is the discomfort will usually go away when your period is over. In the meantime, you can cut back on salt in your diet (it retains fluids...even in your breasts) and there are some over-the-counter pain relievers that are specifically for menstrual pain. Ibuprofen (found in some standard pain relievers) can also do the trick. If your breast pain is really bothering you, or happens all during the month, not just before your period, think about cutting back on caffeine. Remember: Coffee's not the only caffeine criminal. Unfortunately, chocolate has caffeine, and so do some teas (non-herbal and green teas) and lots of sodas. Check the label. If the pain still won't go away, see a doctor, who will probably recommend you take Vitamin E supplements or primrose oil.
Read about breast pain and learn about normal breast development from experts at BeingGirl.com.