Is this behavior from my girlfriend proper? How would you deal with this?

You are not too uptight and her behavior isn't something you should have to "deal with". What she did was horribly wrong. She either craves attention from other men or thrives on making you jealous, and deep down, you know her actions won't stop at what you described that night. Anyone who completely disregards their partners feelings and acts like a tramp is going to bring them nothing but heartache.