What Is The A Burn Mark In The Underarm?

My daughter (14) has a small mark underneath her arm, just below where she shaves. It is about the size of a peanut and the shape of a doughnut, the middle circle being skin, the outer ring being flesh, looking a lot like a burn, but not one. It won't heal and she can't put any pressure on it or even stretch her arm too far. She has never shaved over that area so it can#t be just a broken spot. She is too embarrassed to see a doctor. Do you know what this could be? Thanks, Jenny x