PS: I forgot to sign with my name. It's Katrin.
Dear aubry_ann,
I am glad that you are writing here, and asking your question, and concern. It does not matter if you are 13 or 95.
You should really have this checked out with your regular doctor, or pediatrition and whom you know and he/she knows you. It is the right thing to do.
You should also definitely tell your parents, or mother. If you are too ashamed to show your breast to your mom, you don't have to. All you have to tell her is that you are concerned about a sore you got on your left breast and that you want her to schedule an appointment with him/her as soon as possible.
You do not have to have to justify yourself, or even assume what she may think about how the sore got there. Chances are that she won't even think anything like that you messed around with a boy, and you should just assume exactly the same, as this is true. (and if it was otherwise, you also do not have to justify that)
I really feel for you and I would have been scared as well at your age, but this is normal and doctors see and evaluate problems like yours, or similar, all the time and you do not have to worry about anything and just calmly tell your mom and ask her.
This is not cancer, and most likely having the sore rub against your bra has made it worse. but whatever it is, this can be simply treated with maybe an antibiotic cream or pill, or whatever organism is causing it, like a fungus?
Relax and just do it and I will be thinking about you?
All the best, and please let us know if you want to of course, only, how it went.