Have you had repeat testing including exam when the rash first shows, before they crust? Typically std testing may false negative report if it has been done too early after exposure. Repeat testing while you are symptomatic needs to be done, along with sampling of excretions.
Honestly, I don't know of std's that itch. A herpes sore hurts. Not itches. Have you been swabbed for simple things like overgrowth of yeast? And we can get contact dermatitis or general dermatitis from an unknown origin in the genital area. If you have an autoimmune issue, you can get this. Does anything like vagisil wipes help or witch hazel wipes? If you haven't been swabbed as well for infection, you should be. The rash won't go away until the infection is treated. So, I'd see your ob/gyn for this types of analysis. It doesn't sound like an std to me but more alone the lines of one of these other possibilities. Let us know.
It could be and probably IS completely unrelated to oral sex. So, what about going to a dermatologist? They are good for this type of thing too since you ruled out gynecological issues such as discharge issues (yeast or bacteria). Rashes are their specialty! I'd do this as your next step!