Welcome to the forum. Thanks for your question.
You needn't be at all worried about STDs. There simply are no STDs that cause a recurrent rash of the sort you have described. I'm sure herpes is on your mind because a) it's in the genital area and b) it is recurrent. But herpes is impossible, both on the nature of the rash as you describe it, and because of your negative blood test results.
There are many, many dermatologic conditions -- perhaps a hundred or more (but I'm not a dermatologist, so that's just a wild guess) -- that could cause this sort of thing. Happily, most are not at all serious.
All these comments also apply to the itchy buttock lesions. My guess is that the two conditions are unrelated.
My advice is to see your doctor; or if s/he suggests (e.g. if not up to speed on common skin rashes), perhaps a dermatologist. In the meantime, no worries about STD.
My final comment concerns your periodic testing for HSV and desire for "the most accurate results". If that indeed is your goal, you won't have any more IgM antibody tests. The HSV IgM tests are very unreliable; you are risking a false positive result that will be alarming while you wait for confirmatory tests that show you don't have a new herpes infection after all. For more information, see the thread linked below:
http://www.medhelp.org/posts/STDs/hsv-2--recent-vs-not-recent-infection/show/541451
I hope this has been helpful. Best wishes-- HHH, MD