I've been having somewhat of a similar problem, for about a week and a half now I've had soreness in only one breast. It feels like a bruise, but otherwise no other changes. I'm 20, shouldn't be getting my period for a couple more weeks, and at this point, completely lost. I've tried to give myself a breast exam, but I'm not really sure what I'm looking for..and it's also too sore to really be able to press on to check for anything.