Hormones are a likely cause. Even if you're on birth control, your hormones can vary a bit month to month. Some months, my breasts hurt all month, some months just before my period. It just depends. This is especially true if I manage to miss a pill at the start of the new pack (I take mine continuously for 3-4 mos. at a time.).
How long ago did you start on the birth control? When I stopped taking mine, and then started again after a couple months, my breasts hurt too.