What Causes Swelling And Itching In The Vagina After An Intercourse?

Hi, I have only had sex with one partner, but during "a break" he has slept with someone else. After having sex with him after her, without a condom, the following day I woke up with a constantly itching, swollen and red area. I have also experienced dark brown discharge, which i originally thought was spotting as I am on the pill and this happens quite often to me. I cannot think of any change to washing powder etc. If you could indicate some reasons why I am experiencing this and what to do i would very much appreiate it. Thank You