Make her choose, this is a risky situation. If you don't do something about this you might just get hurt in the end. She's like playing 2nd fiddle here.. nobody can have the best of both man. It's like unfaithfulness in the making. Either you stop it now or bail out. What are you her secret boyfriend? that's fucking insane man.

Ask her "whats the f'n deal?"

Well, maybe not that forwardly, but the only way you'll know for sure is by asking her.