Why does this bother me so much?

So my ex girlfriend and I have been broken up since December. I've been totally fine with it and saw a couple people since then but its never worked out, who cares right? My life was good. New job, getting in shape and doing much better in school without the distraction of a girl.

This was until a couple days ago. She had been talking to me fairly excessively for the last couple weeks, asking me what was new and such; clearly alluding that she had some new development in her life that she needed to share with me. She had met someone new, and felt like I needed to know that because there was an upcoming birthday party for a mutual friend and she wanted to bring her new boyfriend and wanted to make sure I was ok. It stung a bit but I'm a big boy and I can handle it.

Just to give a bit of extra info, I got into a bar fight over her once previous because some guy slapped her.

I spent the entire party staring at this guy wanting to kick the living shit out of him and I don't know why. I broke up with her before and was happy about it but now that she's with someone else it feels like the weight of everything is coming down on me right now and I want her back so bad.

I know its stupid, and its what I want but not what I need but we live in a fairly small community and I'm not leaving for university until next Fall.

So Reddit, what the fuck is wrong with me?