@Admin: That's a really hot explanation - a combo of (a former male I take it) feeling "OMG I just let a man fuck me! (and I liked it!)" and "OMG, am I pregnant?"
Wow, this is in many ways my ultimate fantasy, too - to become a woman, give up my virginity both that erotically and submissive (she obviously wants it, but also given their positions and his muscles he could force her), and getting filled with his seed... maybe being impregnated on her wedding night, maybe turning into a woman, becoming a wife, being deflowered and becoming pregnant on the same day...