I currently have a big brown spot on my arm that starts under the armpit and moves around by bicep. I was told it was Tinea Versicolor because of the amount of sweat I endure from sports. However, I applied Selsun Blue and it stung like crazy, but did not disappear. When I apply Selsun Blue now, it doesnt sting anymore, but it is not removing the spot. This spot is smooth and for the most part is in tact. However there are a few outlier spots close to the main spot. I have had it for 3-4 years and a medical doctor thought it was a birthmark. I dont remember being born with it and I dont think a birthmark should have a reaction to Selsun Blue like it did. Any advise on what this spot might be and how I might go about getting it fixed.