I'm going through my girlfriend's phone
Why? Are you looking for something in particular?
Yes, I'm worried that she is cheating on me.
Your girlfriend will most likely not be super happy about you going through her phone. You are better off talking with her instead of making her think you do not trust her.
Nah, I'm not going to talk to her and will just keep going through her phone.
Just understand that you are risking your relationship by snooping through her phone. If you feel you can not trust her  then let her know why.