Ok...
1. Do not confront her!!! She will just come back at you with "Are you jelous?" and you will look really insecure. At which point she will start having doubts about you.
This is not the type of behaviour you want, but you can't go around trying to control everyone. Which brings me to my second point.
2. Start talking to other girls. Don't ever show her that you're jelous of what she is doing. The reason is that even if she wants to hook up with other guys, you'll have your pick of the girls as well.
Confrontation in this situation is the worst thing you can do. You're trying to control her, and she'll see right through it.
You can't control what other people do, but you can only control your own actions.