Breast soreness tends to come early in the first trimester, and is one of the first signs of pregnancy. The growth and changes in your boobs — necessary to allow baby to feed after arriving — can cause major discomfort. Increased blood flow (caused by surging hormones, what else?) to your chest is probably making your boobs swollen and sensitive to the touch. The soreness should fade after your first trimester, though your cup size will continue to increase as milk ducts and milk-producing cells form.
Your best defense? A supportive bra. Take a professional bra consultant up on their offer, and get an expert fitting. Make sure to leave some room for growing!