Lump in breast.

I'm 15 years old(nearly 16) and had found a lump in my breast quite a while ago but ignored it and assumed it was breast tissue as I'm still developing. But now I have a tight pain in my right sided armpit , the same side as the breast with a lump. This pain runs from my lump and through the armpit. Any advice?? Also the nipple of that breast is extremely painful to touch. It's been on my mind for a while now. Thank you.