Your problem is that you make it easy. If there is something that you don't like, you need to tell her. Now, she shouldn't be blocked from hanging out with her friends (even the male ones), but if you are insecure about her flirting with other guys (and again, let's face it, even with your approval, you are uncomfortable about her being with other guys), you need to tell her because it wouldn't be fair for you to be constantly tortured with the thought of her being with other guys while she flirts around, KNOWING that it upsets you. Just sitting around and approving of something that hurts you only makes it easier for her to walk all over you (and I think she knows she can).
Again, I can't tell if she's actually cheating on you; some girls are the teasing type and she might just be one of them. However, since you are a sensitive guy, you shouldn't have to put up with this, even if you like her. Some guys can handle their girlfriend's flirtatious nature, but you can't. If she can tone it down (and I doubt she could; some people never change), it wouldn't be too much of a problem.
But if she can't, well...your relationship is going to get pretty strained (if it hasn't already).