I'm feeling really nauseous right now
What were you doing earlier that made you feel nauseous?
I slept with a stranger.
While sleeping with a stranger is not necessarily the most sanitary thing to do, it doesnt really make you nauseous. Maybe you eat something earlier in the day that made you sick, or maybe you just happened to be sick in general?
It's because I was sleeping with a stranger.
Well if casual sex bothers you maybe you can wait until you find a relationship. You should only sleep with the people you feel comfortable with.