Am I the problem?

Some background: my boyfriend and I have been together for 2.5 years - mostly long distance while I finished school with long summers and quarter breaks spent together.

We've been having some problems building up for a while, but I'm now staying with him for an extended period of time (1 month in right now) and things aren't going how I hoped.

I'm all in, but he seems hot and cold. Things seem to be going well for a few days then he drops how he's going out with his friends or on his own and doesn't invite me along - whereas before he would have. If I ask to come along he'll blatantly let me know he wants to go by himself. When I get upset about it he makes me/my reaction out to be the problem.

Am I the problem? How often is normal for a guy to go out on his own vs inviting you along? Do I just give him his space and see if things improve? Ughhh