Yes, it's a sign of pregnancy if your bits go purple, all your blood vessels are engorged. It can have one of two effects...either you feel really sexy because the blood supply increases to all that part of your body and your orgasms are the best you have had in your life, or it becomes so swollen and engorged that sex really hurts...charming.
I, too, have been experiencing very sharp pain just behind my lady bits when I walk...I asked the midwife at the hospital about this and she said it's caused by the Symphisis Pubis joint loosening up because of the hormone relaxin in your system. Your pelvic joints are all loosening up to allow passage of the baby out of your body. It can be really problematic and some women have a condition called Symphisis Pubis Dysfunction, which I think a few people have started threads about on here... horrible, isn't it?
Sue