Is my boyfriend hiding something?...

I'm getting kinda worried. Cause, about a month ago he used to always let me be on his phone for playing games and what not. And he sometimes went on mine. Now, lately he's super over protective over it, taking it with him everywhere and not letting me on it anymore. I respect his privacy and everything, but why all of a sudden do this? I don't get it. We've been together for over a year now and I'm 13 weeks pregnant. I have tried talking to him him the coversation doesn't go past "nothing's going on." And it's been going on for about a month now.