It's just you growing. Also, it hurts for all of us, on our period or not. It's an unsecured bra that doesn't hold your boobs and so when you run or jump around the kinda flail around. Try getting tighter bras, but not to tight. If you ever plan on having a baby and plan to breast feed, and you have bras to tight, it won't give the right nutrition to the milk. It will also stunt the growth of your boobs. If you bra fits perfect try tightening the straps more or hook the together to make an X on your back
Well if you already have your period then it's just a typical symptom pre or post your menstration. If not, then you're probably growing, as you said, so it probably just sores.