brown, shadow-like ring on daughter's chest

Hi. I just found out that my 13-year old daughter has a brown, shadowy ring around her left areola. She told me she's had it for a month now. It is not tender. There appears to be no rash, she says it's not itchy. She feels no lumps either. She plays soccer. At first I thought it was a bruise since the girls play soccer pretty rough at this age. But she hasn't noticed the light brown ring changing all that much, whereas a bruise would eventually start to fade. She has not started her menstrual cycle yet. She has been very emotional lately. Could this ring be the result of hormonal changes? Thank you, in advance, for your reply!