What was the story she gave that didn't sound believable? I have learned to not always think the worst. Because whenever I feel like something is wrong and I go snooping or asking what's up it turns out that he was being secretive because he was trying to do something nice for me. Like the other night my bf went to the store and was gone way longer than he should have been. So I started to get nervous thinking he wasn't really going to the store. He comes back and has a bouquet of flowers and that's why he was gone so long.

How long have you guys been together? Has she ever done anything to break your trust before? I think if she has never given you any reason to break your trust then driving to her house was a little overboard. And I think if my bf had followed me to see what I was doing I wouldn't tell him even if it was completely innocent just because I would think "you don't trust me anyways". If you openly show that you don't trust her then she probably feels like you're accusing her of something for simply not responding to a text. You need to bring it up in a non-threatening and non-accusatory way. She will be much more likely to open up about what was going on if she feels like you're concerned about HER rather than yourself. If you say that you're worried about her well being rather than whether she was sneaking around behind your back, she's going to be more open with you. But then again, that's not the truth so.....