My boyfriends having mood swings?

My boyfriend use to be extremely loving and caring. It is a long distance relationship so I understand sometimes it's too much. But we've known each other for about 8 or so years. Past few months he's been getting angry at little things, going to hang out until 1 in the morning without telling me what's going on. He doesn't reply to me like he use to or play games or Skype or just talk to me how he use to. Everytime I ask if something is wrong he just tells me no gets or mad at me. I just want to maybe get an understanding of what may be going on. The only thing that comes to my mind is cheating.