I am 40 weeks pregnant. My vagina is really sore and feels swollen. Why is this?
I'd suggest that you have your provider take a look just to be sure. The most likely explanation from what little information you describe is that you have vaginal varicose veins.
They are uncomfortable, but usually not dangerous for the pregnancy. There are lots of things that you can do to decrease the discomfort if that is what is going on.
Good luck, and have a wonderful labor and delivery!