After having sex, my stomach started hurting, i went in the bathroom and ended up throwing up. it has happened twice, what could be the reason?

Vomiting: Typical sexual activity doesn't usually cause those symptoms and it could be coincidence or abdominal muscle strain which in turn caused nausea and vomiting. If sex was the cause you should try a different technique or position. If these symptoms become more prevalent at other times or still occur with sexual activity, have a physician or your gynecologist evaluate you.