Boobs hurt...!?!!!!):?

My friends little girls talk to me about EVERYTHING so they talk to me about they're bodies and puberty and periods and everything. One of the girls is 12 she wears a 34 almost a 36 B. Her left breast started hurting her randomly really bad it was on the bottom below the nipple she described the pain as someone pinching it from the inside or as someone was punching her in that one spot. She said her boobs had been itching and hurting. She has had her period for 2 years now. What could that be??