Why is my phone sleeping for no reason

So, when I first got the phone, it would sleep in the middle of me doing something, then turn back on. It happened, say, every 30-45 seconds. After about a month it stopped. But now, it's doing it again. It takes off my wallpaper and replaces it with the original one. It's very annoying, and I don't see any reason for it to be happening.

Thank you.