that could mean more blood surply/circulation in that area, nothing serious, but I don't think that it, i think that is the normal color it should be...
It doesn't sound that unusual, did you look at it before the sex and it looked different, or are you just looking at it now? There are some good websites with drawings or pictures of vaginas that you could check to see if yours is in the norm (vaginal lips vary a lot). Google "Betty Dodson" for an artist who has drawn in this area for years, and her site has sort of become a reference for women.
If you have any symptoms indicative of a sexually transmitted disease, of course go see your ob-gyn.
Good luck!