I would think if it was related to the birth control it would have happened within the first six months to a year of you taking it. When I first started it put a few pounds on me, including my boobs, but that was pretty much right away. Might be a combination of your metabolism slowing down with age and the different eating/physical activity habits you have now that you're out of school. Or maybe just some later in life development - I was a size B in high school and now I'm a D..perhaps your cup size just wasn't quite finished growing.