I too, have the same problem. It took two gynos , but one dermatologist to figure it out!. I had every test for std's and also a biopsy done as well. Then I was sent to a vulvar specialist (yes, there is such a thing), who also has a dermatologist present at the first appt. It took the derm 1 min to diagnose the problem.
So, the diagnosis? Psoriasis!!!!! The best treatment I have found is very expensive and an rx. Pramasone cream. I hope this info helps!. Just know that you are not alone!
Take care.