What Could A Rash Around Anus Be?

i am 54 and work the construction field , a couple days ago i got a rash around my anus about a 4"diameter ring , i read about the shingles but the info did not mention this area . it goes away by morning and i work in an outside environment in the mid to high 90's i had it for 2 days and when not working it doesn't show up?i am 5'8 228 lbs in good health