What Causes An Itching Sensation On The Vagina Post Intercourse?

Hi, I just came from the doctors and ran some tests. Abou two weeks ago, I was having sex, maybe a bit rough too, and during I noticed an itchy sensation like when I need to shower or something but assumed it was just the sex. A couple days later I get the same itchy sensation when I change into some polyester undies. And today two weeks after having sex I masturbated and began drying out and stared to feel the itchy, burning sensation again. When I checked I noticed darker color on my left labia majora - a red to purplish spot - no bumps or crusting and on the inner side a swelling. i also had a stiff neck the last couple days. The inner labia majora swelling has gone down in a matter of hours but I m wondering if it s anything serious. I also used petroleum jelly a few days ago on my hemorrhoids near the area. Please let me know. Thanks!