As above.
It seems like she got bored of you, maybe because you got too comfortable or maybe because she wanted to experiment. If she had only slept with you then it must have been appealing to go out with her "bad" friend and have the fun she was.
Bottom line is now she has broken the taboo of dumping you for a while to go fuck around with her mates. Plus it looks like she is using you as a rebound, if she got dumped then for sure she wants the guy back if only to dump him and be in control.
Move on, go through the tough spots with mates, fuck many beautiful women and have fun. You are now free from shackles, have you ever heard the expression, "a pram in the hallway destroys creativity"? You don't need women to feel good except in bed/car bonnet/back seat/etc. Save that shit for when you cant be arsed to go out and chase skirt. Be young and have fun
Best of luck bro