Why did she immediately start hooking up with other guys?

She probably started dating other guys because she was with a guy for so long she doesn't know how to live without as guy so it hard for her to be alone. She might also be trying to make you jealous so you will want to go out with her again.

Its called rejection. You just rejected her, and she needs to know she is still wanted by a men. She is am sure still in love with you. If you want her back you need to open the lines of communication but be sure this is what you want. don't real her in and the cast her out again.

Yeah it's a bad idea, you don't seem to be so into her. what's wrong? not enough fish in the sea after all?

She was just rebounding. That doesn't mean that she doesn't still love you- if you want to get back together with her, then I would tell her.

Well the sad fact of the matter is this, women can get over a break up way faster then a guy, they have tons of friends to get them through their hard times, and us guys have what a few close friends and a bottle of jack, while we are planning on how to get them back, they've already found a new guy and its too late she's over you, happened to me man I mean you can try and get her back but chances are you'll break up again, but if she's already hooking up with other guys she's over you, my advice move on.

She went and slept with a bunch of people right after you two broke up, yeah it's a terrible idea to get back with her.