What Causes Cyst In Breast?

Hi i am 13 years old and i wanted to ask about why i have cysts in my breast .im scared it might be something serious and im also scared that my breast dont grow no more.my nipples are about a penny big and i know that they have to grow more.i had my mamography on september.but my doctor cant check them because she's out sick... they hurt sometimes and thats what worries me the most.