Why are my boobs getting smaller?(teen)?

I am 15 and I had my period for the first time when I was 11. I have always had a weird period like it could last 3months non-stop. but recently it has made my boobs swell once I get on my period. unfortunately (more) recently after each period and swelling and after the swelling goes down they keep getting smaller than before. I was a small to a medium 34c and now a small to medium 34b. I have not lost or gained any weight. I would like to take birth control but my mom, a holistic nutritionist, thinks it will kill me. please help!!