I would ignore the body language, to be honest. Every single thing a person does could mean 9 million different things, but we won't know which one it is until we ask them verbally. He could have some sort of illness, he could have fallen over, he could have (blah blah blah). Anything. Body language is a really flakey thing to worry about - not worth the time and is inaccurate.

I would ask him outright what the problem is. If it is important enough, he will tell you. If he doesn't, then its nothing important so don't worry about it. Probably has nothing to do with you - could be something that happened at home.