It could be that you simply scratched yourself there while showering or dressing. I would advise not picking at it though to stop a risk of infection forming. To put your mind at ease however, it would be best to get it checked out by your primary care physician. Some breast cancers do begin to show through the tissue as a scab, but typically only when it's far advanced. I don't think you have anything to worry about, but it's always better safe than sorry. Good luck to you, and please let me know how you've made out!
Try not to pick at it, put some vitamin E oil on, vitamin E oil is a healer and good for the skin. If this does not help, or gets worse, then see your physician, he/she might refer you to a dermatologist.
It might be because of your period because i get those and then they go away. but you have to stop picking at it or it my not go away