Rash on Rear End... HELP!!!

For the past few years I've been experiencing a rash every once in a while between my "cheeks". It typically lasts for about a month before it completely goes away. It's itchy, pink, bumpy, scaly and gets flaky towards the end of it's cycle and usually is about two inches tall by one inch wide and is always in the same spot. FYI, I never get blisters with it. All of my life, litterally since I was a child, I've had very sensitive skin and have had problems with rashes. I've been tested for herpes and all the tests have come back negative. So, I know that's not it. Does anyone have any clue as to what this could be. I probably get it a couple of times a year and it's more frustrating than anything. I should probably go to the doctor about it, but I'm embarassed. Thanks!