Rash on buttocks

Hi Dr,

Just today, a large rash appeared on my upper buttocks. It wasn't there yesterday and then mid way through my day i felt a little discomfort and i saw it. It goes from the bottom of my underwear line down a little past the beginning of by buttocks. It forms a straight line at the top when my underwear line is. It is kinda splotchy, red, dry, but it is not itchy or painful at all. It is just a little uncomfortable when sitting down at a slant.

I just worked out the other day, and I hadn't in a while. I also tend to sweat a lot. I am really concerned, and I am not sure if I should see a doctor or not. Could this be herpes or an STD? The only sexual activity I have had is hand jobs.

Thanks.