What is this little flap of skin right at the vaginal opening? (photo)

I have had this tiny flap of skin as long as I can remember. It is hidden usually just inside the vaginal opening; if I don't use my fingers to spread the labia then it is not visible. As soon as I spread them or widen my legs, ie for sex, you can see it. It is not sore or painful, ever, but slightly embarrassing. It is less than a centimetre long and a little less than half a pencil thick. I have never seen another vagina with this thing in it.