Breast cancer at a young age?

I've been noticing my left boob never fills out my bra as much as my right one (the straps are tightened evenly) & I looked in the mirror today and noticed my right boob is in fact, bigger than my left one. I don't know what a lot is considered but it hangs down farther than the left. I looked it up and people say different things and someone said it could be breast cancer. My mom is a nurse and said I shouldn't have anything to worry about but it really does worry me. I'm 14 but I look a lot older than I am. Could it be anything serious?