Why is my boyfriend being so edgy? Is there another girl or is he just upset?

Ok, so this is somewhat of a repost of "Do I have serious issues or is my intuition telling me something". But that post got sloppy and wasn't well thought out, so moderators, please feel free to delete it if necessary, but please leave this one :)

So here goes: Basically, my long-distance boyfriend of 3 years has been very edgy these past 3-4 days. He's a bit standoffish and conversation has been somewhat shorter than usual, and his responses are short and uninterested (but only at random times during the conversation). When I ask what's up with him, he doesn't want to talk about it, and would rather just go to bed early.

Since he won't really talk to me about it, I clearly don't know why he's been acting this way….So this is where suspicion comes in.

I can trace the beginning of this edginess back to this weekend. So I have come up with 2 possible reasons for his behavior:

1) Sometime this weekend, he cheated/did something he doesn't want me to know about. I think this because he blew me off and got irritated when I texted him this weekend while he was "out with the boys", after he told me that he wanted me to. Then today, he went out to the bar with a friend, until about 10, on a weeknight, which is not typical of him. Granted, it's not extremely unusual, it's just not very common at all. The fact that he's so edgy and defensive, and goes to sleep early really helps make this reason more believable. But…I can tell that he honestly is going to bed...alone. So with this reasoning, it seems like his behavior fits perfectly into the "guilty conscience" category...

2) I mentioned to him this weekend that my sister was considering moving to where he is with me. He didn't say much, but his reaction was clearly not one of excitement or approval. We had planned up until then for it to be just him & I. However, we had no plans to move in together right away, I have already decided that I want to get my own place first. I asked him how he felt about her coming, and he basically just said that he didn't want to have to worry about her hanging out with us all the time. I think this could be a reason for his odd behavior because obviously he doesn't like the idea of her moving there with me very much. Maybe he feels disappointed because he was looking forward to it being just us two, but doesn't feel like he can tell me that because that would be telling me that I can't bring my sister, and maybe he thinks that would make him look like a jerk. While I asked for his honest opinion on the idea, he may feel like my mind is already made up...So with this reasoning, it seems like his behavior fits into the "hurt feelings/upset" category...

So I know that I won't really know why he's acting this way until he tells me, but obviously he won't tell me right now….So I just want some opinions because I feel really really weird and uncomfortable and it's extremely distracting me from school work :/

What is the most likely cause for his behavior?
Why is my boyfriend being so edgy? Is there another girl or is he just upset?
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