Swelling during sex?

Currently 16 weeks 5 days and not having sex very often at all. Maybe once a week or every two weeks. Anyway, today hubby and I decided to do it and while he was going down on me he said he noticed my lady bits swelling up and opening up a bit. I wonder why this is...we didn't do it rough or anything. Have any of you ladies experienced this and is it just due to the increased blood flow during pregnancy?