What Causes Itchy Rashes And Black Marks On The Breast?

My daughter is 13years 4 months, 6ft 3 tall, Black British. She has large breast 38DD. She has a itchy rash on right breast on dark area which has been treated as eczema but still hasn't gone away about 6months. Last night noticed dark marks (looks like a bruise but not) on left breast. I've made appt to see doctor but can't help but worry please advise