I'm actually interested in where this goes @
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Since it's merely perception at this point and I basically agree with the confront-tactic, I'm just going to drop in a few pointers to keep an eye out for.
First off, don't come across as pissed off before you have a reason to. There may (or may not be) a perfectly logical explanation for everything, and remember not to incriminate your GF before she has actually done something wrong.
Second, I always suggest trying to work things out, at least once. Unless for some reason she was doing it for money or something along those lines, it's probably something manageable.
Third, although we as a forum were quick to incriminate your GF, that's only because we heard one side of the story. Take what we say with a grain of salt, because there is a slight bias.
Fourth, if it's not going to work out, don't be an ass about things. You never know what the future holds and it does no one any good to be a dick.
Fifth, when you do ask her about stuff (address, best friend, etc) try not to sound like you are accusing her of a wrong doing. Putting someone on the defensive will only ensure you get part of the truth, if any at all. The best thing you can do is explain your concerns in a non-abrasive manner and coax the truth out. From there you can make a better decision than if you are already heated up.
I really hope everything goes well. Good luck sir.
Federation - That story is the reason why I will never date online...just saying.