Ann, I can't stop wondering about your butterfly experience... I wonder if this happens in any other movements you do? That exercise certainly has some significant rotation/ stretching/ opening movements to it, which may produce nausea I suppose. I hope you'll have someone exam it if it persists or shows up during other similar movement patterns.
I will share here that for years (although not so much recently) lots of side work (like sideovers) would make me nauseous. And here's the odd part... it happens to my daughter, too!
Kristi...yoga makes me EXTREMELY nauseated. I have never gotten nauseated while working out with my pilates instructor...and she friggin' kills me everytime. But I don't know...I'm weird. I honestly think it's the motion (like motions sickness). I do remember getting nauseated during a dance type workout once.