I had the same problem and I finally went to a different doctor. That was the best thing I ever did. I don't remember what they called it or even what they gave me but I thought I was never going to relief and I finally did. So, my suggestion would be get a second opinion.
Is this "rash" on perinium...(between vagina and rectum area? Is it inside Labia? Maybe is its on the skin area it is a fungal infection or a dermatitis in a weird local? Try searching in the dermatology area....also try the expert area for a physician to answer you, but be more detailed in your symptoms.