I can't imagine what it is, and I know your doctor said no yeast infection, but have you tried taking loads of acidophiles? You can't take too much and you can get bottles of it at health food stores and even grocery and drug stores. It's good bacteria like you find in yogurt. It can help a lot! Try it.
Also, have you tried using a free and clear detergent with no dyes or perfumes? Cut out any detergents, fabric softeners, etc. with scent or dyes in them. Try to use some 100% natural stuff. Likewise with bath and body products -lay off of any scented or dyed washes, soaps or lotions. I'm sure you're not douching, but if you are, stop immediately! It's terrible for you. Do NOT use any powders, scented tampons or pads or pantyliners. Make sure your underwear is 100% cotton and fits well -even a little loosely. In fact, I would go out and purchase some 100% dye-free organic cotton panties and see if those helped. What do you use for birth control? You may need something else, especially if you're using any foams, spermicides, films or condoms w/ lubricant or nonoxyl 9. Nonoxyl 9 sets me on fire AND it makes me itch for several days just like a yeast infection! You may have done all of this already, but any of the things listed could be the culprit.