Hi doctor, In summer I noticed a dark part on my back, I thought it might be because of sun burn . but it started to growing, although I never went for sun bath anymore. what makes me concerned is that it s only on my ribs, just getting dark on my back ribs and it s coming toward my front ribs. It is not itchy, or sore. I have no unusual feeling, but it s spreading and getting darker. what do you think it can be? Thank you