If your ex is making things up about you, he's obviously miserable. It's just like, 'Wow, this person really cares to go out of their way to start a rumor about me.' I've dealt with it so much, obviously. The first couple of times, it really sucks. But then [they] just come out with something else. If you dwell on it, it's going to make you miserable. Just move on and laugh it off.
If a guy's talking to you at a club and you're having a long conversation, and then one of your friends comes up and he automatically devotes his attention to her, that's always a sign to look for. They're not always just doing it 'cause they're being 'friendly.' They want to look for somebody new.