I don't know what to think

My boyfriend has been acting hella weird lately. Whenever I touch his phone he immediately snatches it away from me and he's done this multiple times. Then, my friend sent me a picture of him and another girl together. Mind you, we've been together for almost a year and a half and he's never ONCE hung out with another female. And, he didn't even mention he was going out let alone with a girl. Plus I didn't even know her. I didn't know what to think of it so I DMed her and asked if anything was going on, and I did it very sweetly and calmly. But when she replied she was hella defensive. The thing that really set me off was the same night I was shown the pictures of them together she was at his basketball game. I have been really confused lately. He's been acting weird and I don't know what to think about it. I just want things to go back to the way they were.