Girls only PLEASE!!!!!!!!!!!!!!!!!?

I am in 7th grade and I am a size 36A in bras. But in the past few months i found I have been....shrinking. I don't know if it is because I am now wearing wire-free bras, or because its cold out for winter or whats going on...but if you know why please tell me! Will this go back to normal in summer or do I have some kind of growth problem going on?