Hmmm... well there could be a lot of things going on there. How is your relationship any other time when you are not even thinking about sex? Are you happy overall? One thing could be the fluctation in testosterone which happens during and after sex. That can seriously affect a man's mood. Or, it could be a habitual pattern. Maybe something that started and he unknowingly does out of habit? Or, as you said, something could be bothering him mentally. Does he usually initiate sex or do you? Is your sex pretty regular without much change as far as positions or creativity? If it is, then you might try to spice things up. Make things different. Have sex in different places. And then afterwords, try to engage in something positive together. Something you both enjoy doing. This might help to either break the pattern of negativity afterwards or at least take his mind off whatever is bothering him.