Most Helpful Girl
Well in certain circumstances, like if you ended things with him, the person who was left usually feels hurt. So he may be acting out because the break-up caused him to feel insecure and he is taking it out on you.
For me there are always stages in breakups. At first you just want them back, so you're sad and miss them. Eventually you realize they aren't coming back and you start to hate them because they didn't want you. And then there's some sort of closure hopefully and you move on.