What Causes Redness And Swelling In Vagina?

so my vagina has been red a swollen on and off for two months now. I went to my doctor and they tested me for yeast infection UTI, herpes and a couple other std's. everything came back negative but the problem was still there so about a month later i went back to the doc's and they are just testing me for the same things i got tested for the first time. what could be wrong with me?