What Causes Itchiness In The Vagina After Intercourse?

i had sex last weekend for the first time with my boyfriend and it was unprotected! we were both virgins but he pulled out before he came. everything was fine during this hole week but randomly on saturday night i started to get really itchy on the outside not on the inside. i didnt take much notice but it just kept getting itchy and now it wont stop!!! what is happening???