What Causes Vaginal Itching And Swelling?

My lips (down there) started itching yesterday n I kept rubbing it ans scratching it and I noticed today that it is a bit red and my lips swelled? I don't know exactly what it is but I am a active person and I wourk out every day. I do wear thongs n shorts underneath my gym shorts and I'm kind thinking that may be it. But again, I don't know what it is. It doesn't hurt and I haven't seen any unusual discharge.