sometimes your breasts do get larger due to hormone shifts in your body. It is entirely normal. You may also experience more severe cramping, weight gain, and mood changes. there is nothing wrong with any of these. If however your cramps become hard to deal with and your breasts seem really sore you do need to talk to your doctor.
Birth control pills that contain estrogen can make your breasts larger. Those with progesterone rarely have this affect. If you are concerned, talk to the doctor who gave you the prescription and change pills.