Breast Cancer???????

I just turned 13... and im going through puberty. Behind both of my nipples are lumps... what could they be? They are about the same size although the left one is a bit larger. They don't hurt. I heard that your milk glands are developing during this time...so is that what it is? and also...how long should they stay or will they ever leave? I have no history of breast cancer in my family.