Welcome to the homosexual milieu. There is a general sense that, in the absence of an explicit agreement regarding sexual fidelity, that there ARE NO expectations. She could be having sex with those other girls, just as you could be having sex with still others.
This comes down to what the two of you have discussed and agreed, and to trust. You either trust, or you don't. There is no trusting one day and not trusting the next, or trusting her in these circumstances but not trusting her in those circumstances. If sexual fidelity is important to you, then trust her - or leave. No middle ground.
(oh, rats - I guess that means you'll have to decide if sexual fidelity IS important to you)