What Causes Burning Sensation In Vagina?

std?
recently I started to experience a slight burning on just the outside of my vagina. I went to urgent care for an STD screening due to my gyno being out of town. they told me they couldn't physically see any redness inflammation or discharge. and they said without discharge there isn't any way they can test me. I don't believe this to be true but she was a Dr. she gave me diflucan for a possible yeast infection and sent me along. in the next few days the burning has gotten worse and even sex has become painful with my husband. what could it be and what should I do?? thank you, raven