She has a right to privacy, however I do understand where you're coming from. If this is new behavior for her and she used to not care if you saw her phone, then maybe something is going on. But don't let your imagination run away, it could be something simple and innocent like a surprise gift for you or planning some event.
I had a similar situation with my ex, for years she never cared if I saw anything of hers, emails, phone, etc. and I didn't care what she saw of mine, I didn't go looking but on occasion we would grab the wrong phone in the morning or leave our email open and it never mattered. One day she started getting very private always having her phone with her even in the bathroom, changed all her passwords, etc. Turned out she was just starting to talk to another guy and it was escalating, I caught on right before it could have gone too far but the damage was done, she lost my trust permanently and I filed for divorce.
You know her, watch her reaction when the phone rings, especially at odd hours. If she is changing her patterns and her overall way of acting, you might want to look into it, but like I said don't let yourself get out of control with assumptions. I've had friends with spouses doing similar things and almost always turned out to be something innocent, one was planning a party, another buying her husband a car and all the corresponding was done through email and on the phone so they had to safe-guard them.