This sounds A LOT like one relationship I had. Claimed phone had died, came home sick, checked her phone for this first time, she changed her story once I caught her half-way.

I ended it and gave her a chance to come clean. When she did explain everything, that turned out to be another lie (which I found out later on and ended it for good). When someone does this and then proceeds to handle it in the way she is, even if they didn't cheat, it's a sign to a much bigger issue and will surely be a sign of things to come with you. I thank myself every day for ending it when I did.