Why Are My Breasts Sore?

My breasts have been so sore for the past few days and I have no idea why. When I was 13 my breasts felt like this when they grew, but they haven't grown for a year or so, so I thought they were done? I started birth control for heavy periods a few months ago, but it hasn't made me sore before. I've been exercising more, recently, but I was wearing a sports bra. Could it still be from that?