Mine were like that, but about 3 years later. (No period until I was 15, but but I was a D cup by the time I got my period.) The evened out in size as I got older, but one always drooped more. They are much more asymmetrical now, because I had a trauma that caused swelling for a long period of time, and stretched the skin/tissue. Anyway, my OB told me asymmetry (everyone's breasts are a bit asymmetrical, I'm talking a big difference) is often a tissue problem. It's one that can cause trouble producing milk and lactating. (Doesn't matter the size, they can be big or small.) What's interesting about her telling me that, is that I ended up producing nothing. My breasts never even hurt, or changed during pregnancy. If it's a tissue problem, then you can't really do anything about it.
With that said, it could just be hormonal. My 12 year old niece has 2 different looking breasts (I've helped raise her, and took her to the appointment.) and her Dr. was not worried. She said it's not uncommon for breasts to be different sizes and shapes during puberty. SO...it could be something, and it could not be. The "something" it could be, is most likely nothing at all...and simply just differently shaped breasts.