My breasts grew two cup sizes in one week??

So I am fairly petite (like 5'5" and a size zero) and athletic, and I have always had A cup breasts. Over the past week, they have grown (no pain or tenderness) to a Concert cup. I had to get actual bras, because I exclusively wear unpadded bralettes. Is there a reason to their growth?

I am 17, and I am sexually active. It's unlikely that it's pregnancy, because I had my last period three weeks ago, and it ended two weeks ago. Additionally, I am diligent about protection. I am not on the pill. Thanks!