All this secrecy, if a recent occurrence, does sound very suspect and like what some people do when cheating.

I think you’ve done really well for opening up and taking the time to trust someone and if I were you, I would just break up with her. Once any form of trust is compromised, it takes a lot of equal work to get back to the level it was. From the information you’ve given, it looks like she’s guilty from here, and from how she’s been treating you, I doubt she would want to make it work either.

Don’t ask for an explanation, you don’t need one. Just break it off and count it up to experience and don’t let it hinder your next relationship either.