HELP 10 points! is she a rebound or not?

So my ex and I broke up two weeks ago. We just kept on fighting a lot were under a lot of stress because of school and work and it didn't heelp. On top of that we were planning on going our seperate ways (he was going to lv the city for school) and I was going to stay. We tried to be friends but it didn't work out. So I just recently saw him at the mall with another girl and I later found out that they're dating. I just don't get it because I feel I had been lied to. We were in a heated argument and ssaid some prett mean things to each other and he explained that his new girlfriend is everything he's ever wanted and I was a crappy girlfriend practically just rubbing it in my face. I don't understand why he would get into a relationship with a random girl that's nothing like me and isn't really someone my ex would date. I think it was out of desperation because he said that she was there for him when we ended and brought him to parties since my ex was a loner and didn't have any friends. So I was the only company he really had I just don't understand the reason of getting a girlfriend and then leave off to a new city for school, to me it just sounds like a waste of time. So I'm wondering if he's only using her to get over me or did he want to leave me to be with another girl. He also asked me if I cared that he has a girlfriend and so I also think that he did it to get a rise out of me of some sort, or to prove something. And when I mean his gf is not the type he would date I mean that she smokes cigs and weed which is something he hates! And also got out of a relationship as well