Because he now found that his ex girlfriend is prettier than you and prettier than before.
It means the ex boyfriend still likes the ex girlfriend.
She probably did it to make the ex- boyfriend jealous.
maybe he wants you back.
To say: "Just to let you know, I dated this girl before you; but we are through now"
That would really be up to you and her.
You can not do much, except wait and see. If he will leave his current girlfriend, then you can try win him back. But till then, you can do nothing. Sorry.
You go up to your boyfriend and tell him to choose, but if he can't then break up with him. Why go out with a guy who goes back out with his ex if he has another boyfriend.
You do absolutely nothing about your ex boyfriend considering he has a new girlfriend. If he loved you he would break up with his girlfriend and since he has not then he wants to be with her and you need to accept that. Just because you have broken up with your boyfriend it doesn't mean you can just step back into your ex boyfriend's life.
Obvoiusly, he doesn't like you anymore. He would think it would be akward to have his current girlfriend and his ex-girlfriend around at the same time. I mean put yourself in his shoes, if you have a boyfriend, and your ex-boyfriend is hanging around, wouldn't that be quite akward? I would think so. Try to aviod them; find a new man.
be yourself
He had a girlfriend. Her name was Kasey. They are not boyfriend and girlfriend anymore.
I would advise you to talk to your ex about it first and see how she feels. If she doesn't feel the same way, then go back to your girlfriend or boyfriend, or depending on how strong your feelings, it would be best to break up with your girlfriend or boyfriend, anyways.