Cheating?

So lately, my boyfriend had been acting really shady. While on his phone, he'll face me and wherever he goes, he brings his phone with him. I noticed his score would go up by a lot and I told my friends about it. Everyone thought I was crazy. One day, he accidentally left his phone beside the bed so I searched it. 
He had changed a girl's name on snapchat to a male's name not knowing I could still see her username. I asked him about it and he responded, "Shes just a coworker and bought a car from me. We don't talk that much. I changed her name because I rarely talk to girls and you seeing it, you'd think it's weird." Something tells me I shouldn't believe him.
We made up and he told me he deleted her off snapchat. I believe him, but I don't know if he communicates with her any other way. They're coworkers, so I can't stop him from talking to her. But why hide her from me if there's nothing going on.