It may not be cheating, but it isn't anything good. Just the fact that he's keeping secrets from you is a bad, bad sign.
You need to start having some REAL, HONEST conversations with him. Even if you're going to be mad or hurt, you need to ask him to tell you the truth, and do NOT get upset with him. People sometimes act out like this when something ELSE in their life is going wrong, and he may be embarassed or ashamed to tell you about it. Maybe he needs help. He could have a drinking/drug/gambling problem, or owe someone money, or some other thing that he's dealing with that he needs some help with.
But don't put this off: you need to deal wiht this ASAP, in a non-judgemental way, and fix this relationship, or it's going to be over before too long.