I'm so sorry you're having such a difficult time.....and believe me, I know it's not funny! I'm shooting in the dark here, but let me tell you a story about something that happened to my husband, and see if anything sounds familiar at all:
I was dx with celiac disease in 2008, around Thanksgiving. I tried very hard to go gluten free by myself, but still allow gluten-y foods in the house (regular wheat bread and pasta, etc) for my husband and kids, but it didn't work. I kept getting accidentally "glutened" all the time by the tiniest crumbs, which could make me sick for days. So eventually our entire family had to go gluten free; the house became a gluten-free zone.
Being gluten free isn't as bad as it sounds, since almost anything from bread to pizza to muffins to pasta can be made gluten free, and still taste pretty good. My husband was a good sport for a few weeks, and remained completely gluten free until the Superbowl, when he bought some beer. Beer is made from barley, of course, so has gluten in it.
Within a few days of drinking the beer, my husband broke out in the nastiest blisters that broke and turned into sores in exactly the same place you're describing...right at the base of his spine/top of his butt crack. One sore expanded to the size of a quarter, it looked like a crater on the moon. The itching was driving him crazy! It turned out to be a rare form of celiac disease that manifests itself through the skin, called dermatitis herpetiformis (DH for short). He also ended up with a horrible skin condition on his hands, knees, elbows, etc. which involved a mixture of DH and psoriasis.
We were totally surprised....we had absolutely NO idea that he too had celiac disease, since he had never had any symptoms!!
Of course, he immediately went back to being gluten free and took some medication as well, but it took a LONG time to heal (months).
The DH sores turned a dark dark purple as they healed, and the purple "scars" stayed there forever. That's a hallmark of DH, blisters that break and turn purple as they heal, with dark purple blotches that last a long time.
Anyway, that's what I was reminded of as I read your post....sore at the top of the butt crack that itches like crazy and is NOT yeast. Have you ever been on a gluten free diet? Do you know if you are gluten intolerant or not?
Hang in there! I know how frustrating it is to be in diagnostic limbo!
Lyme Disease, Babesiosis, Ehrlichiosis, Bartonella, AIH, Hashimoto's, lupus, fibro, RA, celiac, asthma, psoriasis, adrenal fatigue, pre-diabetic, Raynaud's, hypertension, osteopenia, sleep apnea, RLS, GERD, DDD, severe lower back pain, cubital tunnel, tarsal tunnel, Suprax, Malarone, Alinia, Zithromax, Prednisone, Imuran, Plaquenil, Lyrica, Cymbalta, Levothyroxine, Liothyronine, Atenolol, Cozaar, Zyrtec, Fosamax, Albuterol, Prilosec