What Causes Labial Reddening With Vaginal Discomfort And Discharge?

For the past couple of months, I noticed that there was one spot right on the inside of my Right labia (up toward the clitoris) that seemed a little bit red but mostly the skin was just a bit thicker than the rest. Just this past week, however, I noticed redness spreading to the other labia and then all the way down the labia toward the vaginal opening, where the skin began to swell. I haven't had any itching, just a lot of discomfort. I also have been having some unusual discharge, whitish in color and sort of slimy. I have discomfort in the vaginal opening as well (almost like a warm sensation?). Today my doctor tested for yeast infection and BV and didn't see signs of either of those, but decided to treat me for a yeast infection anyway. I'm curious as to whether a yeast infection is actually possible or if there is something else that could be happening? My last partner was about 4 months ago and I never got STI testing done afterward so I'm not sure whether an STI could be possible too. Thank you.