It could be a yeast infection. They are extremely common during pregnancy.
It's normal :) the further along you are, the worse it gets.
It could be because of increased blood flow to the vagina. You already have an.increased blood flow down there, and during sex your body pumps more down there. Some women swell without having intercourse. It doesn't sound like an infection though, especially if it only happens after sex. Other than the increased blood flow, I'd check.for anything you can be allergic to.