I couldnt believe this question when I read it. To me it sounds to stupid to be real, but then again there are some very stupid people in this world.
I'm gonna just say it, she's cheating on you, dumbass! I'm surprised you haven't put two and two together yet. Dump her and find someone who will appreciate you for you. You don't need that garbage, and that's just what she is.
If she cared for you, or had any kind of respect for you, she wouldn't do this to you, or at the very least she'd be honest with you. You can never trust someone like that! Ever. If you did this to you once, she will do it again. And there's nothing you can do to stop it.
"Whoever can be trusted with very little can also be trusted with much, and whoever is dishonest with very little will also be dishonest with much."
Remember that a person's actions, not their words will tell you how they truly feel about you, and who they truly are as people, and that applies to you as well.