What Causes Dark Spots On Upper Back And Underarm?

Hi, I am going to a doctor tomorrow but I wanted to get an idea first. I have these two spots on my upper back, close to my right underarm. They are darker than my regular skin tone but not to much darker, they look more like a brown bruise. They are side by side and are circular but not perfectly, in fact one has a area that looks more like a tail pointing downward. I have only had them a couple years. It originally started as one then the other appeared last year. What could this be?