Most Helpful Guy
I recall being very sour towards my high-school girlfriend a few months after she broke up with me and I realized that she was stringing me along for a few months and toying with me. Depending on how the relationship broke down, it could either be resentment or "unsorted business" that you never got to sort out, or it could be that he was a genuine prick.
Another possibility is that it is a defence mechanism. I remember when she broke up with me, that it felt like a giant hole was blown into my being, and I had to fill it with something else. Luckily I turned to books and self-improvement, but I have seen other guys pretend that nothing is wrong and suffer over a longer period.
Unfortunately, a lot of people (both genders) don't realize that the best the best thing to do at the end of a relationship is to segregate yourself from the other person and spend time with your own friends (a reason why I am adamant on me and my girlfriend having our own friends).
Anyways, hope it answers your question, and I hope you feel better soon.
Most Helpful Girl
He's being deliberately cruel. The girl is probably an ornament for his game to make you upset. She might find the same thing happen to her.