It's the pill. When I went on the Pill, for about the first three months my boobs were really sore. It's because the Pill makes your body think that its pregnant, so your boobs get sore because they are growing in preparation to feed a baby, basically.
You need to get measured and fitted for some new size bras. I went up a size and a half (awkward indeed) in the first few months of being on the Pill. And they are still growing away.