Is my boyfriend cheating on me?

So lately my boyfriend has been hiding his phone from me. Like, changing the passcode, taking it with him wherever he goes, if I even jokingly take it to take selfies he takes it back immediately. There was one incident where I was at his house, and he had to go outside to get something, and he went to walk out the door, paused, and then got his phone and left. I've talked to him about it and he said he was hiding it because he was planning presents for my B-Day, but now it's come and gone and he says he's planning for Christmas. I also know that he likes to do text based roleplays on kik and they're often with other girls and revolve around romance. He's told me that they're basically stories and that he keeps them strictly PG, but I told him awhile ago that I really wasn't comfortable with him doing them and he said he would keep romance out of it, but I just found out he's being doing a romance one with some girl named Nicole. Am I wrong to be suspicious? We've been together 10 months and I trust him but he's acting really weird to me.