I've ran into this problem myself. Sometimes it's hard when the other person isn't around...I mean you can't not go out and have fun just because you b/f g/f isn't there, that's no way to live. If you're expecting her to not go out at all, then you're in the wrong. But if you're honestly concerned, and you have reason to believe,that she'll cheat on you then maybe you should be worried. How secure is your relationship? Does she know that you feel this way? And just in case you're thinking about it, don't get back at her by trying to make her jealous too, it'll never work if you try that.