Was I wrong to get upset over this?

I was friends with a girl for about a year everyone tells me she was a slut and I ignored them but then after she got to the point of calling me her best friend and telling me stuff then she stopped telling me stuff, she started ignoring my text messages about half the time then and i got a job so I asked about meeting afterschool when I was done working but she just made up excuses so I start riding the bus again to save gas and she sits with me and we talk but then one day she walks past me and sits with some punk who picks fights everyday who she just started talking to the day before when I wasn't on the bus, she already was talking to a couple guys about dating and a few friends say more like 5 or 6 and I got a mad she ignored me but she said she was really sorry and didn't notice me but on the way home she sits with him again after looking at me and then asks me whats wrong and gives me a pouting look like shes sorry because she knew it bothered me but then I start talking to her on facebook and after saying hi she ignores me asking why she wasn't sitting with me and posts "I think he isant going to talk to me anymore :(" and I try asking who and she tells me "who do you ****** think??" but goes back to ignoring me so I finally get her to IM back and we argue but decided it was my fault for getting upset the next day I decide I don't want to fight with someone I have to see everyday so I try talking to her and she starts threatening to tell all the guys shes talking with and get them to beat me up she is only 13 but she flirts with every senior in the school so I called her a slut and walked off.. did I overreact? I know she has a right to talk to other guys it was just that supposedly she was dating a guy so the other guy would have been only a friend which considering he only knew her a day kinda hurt she already was talking to him instead of anyone else on the bus who were her real friends myself included. and I have been told by 10 former friends of hers and an ex bf of hers that she ditches her friends and stabs them in the back so I was wary.