Hormone fluctuations can cause your boobs to become sore. Mine were sore all the time starting about a couple weeks before my period before I started birth control, but afterward, they weren't. Now (after about three years on the pill), I'm beginning to experience it again sometimes.
It could be that the pills are affecting that. You said you're on day eight of your pills. Does this mean you just started taking them for the first time eight days ago? Birth control pills are known to increase breast size as well as cause soreness and such for some women. Not everyone is effected the same way, however.