What Causes Vaginal Itches After Periods?

21, 5'4, 132 pounds
yes, a few months ago I woke up and it hurt really bad when I urinated so I went to the er and they said I had a bladder infection and put me on meds, then about 2 weeks later the meds caused a yeast infection which was the first time I had ever had one. So I went to my reguler female doctor and she put me on meds for the yeast infection and it cleared right up. But a few weeks later on my right second iner lip of my vagina there is only one spot that itches and it gets better but starts itching again before and right after my period. When I rty to have sex it becomes very red and swollen and hurts very bad! I have been testes for the most coomon std's and everything came back fine, what could this be????