Dont think i should be angry about this?

well theres a girl that i thought i was over, it is the only girl i have ever really fell for. and we are still really good friends and hang out quite a bit and i found out last nite that she is going out with her ex. i thought i was over her but i just got really pissed off.. like i dont get why she is going out with that guy again. and she hasnt even said anything about it to me.. she tells me everything and hasnt even mentioned that she stgill talks to him..am i aloud to be pissed?.. and why the hell am i jelous when i dont even want to go out with her anymore.. and why jelous of soemone like him.. i dont get it!!! grrrr lol