Mysterious Perfectly-Circular Non-Itchy Rash

Just discovered a circular rash on my bottom and am bit uncertain as to what it could be. At first I thought it could be a ringworm but it doesn't seem to fit the description as it is not raised, there are no bumps and it doesn't itch at all or feel any different to the rest of my skin. If I didn't spot it, I wouldn't have known I had it. No symptoms to report other than feeling a bit lethargic but I don't think the 2 and related.

Would be very grateful if you could help me identify what it could be.

Photo:
http://imageshack.us/photo/my-images/821/4 6799962.png/

Thanks.