It is probably nothing but you should go to the doctor to put your mind at ease.
You have your young age going for you, it is I believe so rare that someone as young as you develops breast cancer. I have known a few women in their late teens yearly 20s that felt something that ending up being nothing.
I am a breast cancer survivor and want to you to know that the unknown is almost worse then the known. So go and get some peace by seeing a doctor.
God bless, and know that jesus cares greatly for you!!!!! :d
millie