Hi, I Have Had This Itch That Is There All

Hi, I have had this itch that is there all the time under were my bra lays but on my rib. It also has a dark color there now. I put cortisone cream and when I did I felt a small lump that is positioned right on my rib. It does move when I rub it. But I can only feel it when I am placing cream there. I'm worried it may be cancer. It has been itching for months now and just recently became dark in color. I think from me rubbing and itching it. What do you think it can be?