Could he be cheating?

My fiance doesn't seem to want to spend much time with me anymore, when he's home he hangs out with a friend. He started shutting his phone off at night or putting it under his pillow. He runs off to do some errands he tells me will take 10 minutes and comes back over an hour later. He also doesn't answer his phone at all anymore while he's at "work" when he used to all the time? Should I be concerned?