What Causes Burning And Irritation In Vagina?

I have been suffering from vaginal burning and irritation for about a month now. I went to my gynecologist who treated me for a yeast infection- no relief. I then went back a second time and she tested for bacteria and stds but found nothing. She also found no physical signs when examining me except for the slight redness and irritaion of the labia. The pain occurs when my labia are touched, like in sex. It has gotten to the point where I can't even have sex anymore. What could this be?