Look... its possible that she's cheating on you, just as its possible that everyone's other half is cheating on them. You can't do anything about it right now, so if this doesn't work out, then maybe you should plan on what you're going to say when she gets back, i.e. a cool, rational chat in which you both share your feelings about the situation.
On the other hand, perhaps she isn't answering because she feels you're being paranoid and are 'checking up' on her through mistrust. Its possible. Basically, there are a lot of possibilities; some of them not so good. Just keep calm, don't send her a barrage of messages, and wait to see what happens. If she *is* cheating, then this is the way to come through with dignity.