It's cool. I've been in similar situations in the recent past... I have a more stable situation now, and I am not saying its perfect or anything like that but it definitely does have a different feel. Take your time to find a partner that considers your feelings before they act. My girlfriend would never do something like this, not because I tell her not to, but because she wouldn't want to cause jealousy and turmoil in the relationship. Instead of drinking and partying with strangers, she would come visit this weekend while I work - and then make plans for the two of us to go out and party together with said strangers when we are both available. She would expect me to behave the same way.

As for the breakup, one more tidbit of advice. I really think you are better off not confronting her about it. You're going to tear yourself apart and look weak if you break up with her and use this weekend as an excuse. She will paint you as the controlling dickhead that broke up with her for not letting her have fun, yadda yadda yadda. Girls twist things. Just delete her from your phone and disappear from the face of the earth. She has hurt you, she knows what she did, and she can stew in her own mistake. You'll feel better than if you talk to her and let her either make you doubt your choice, or rip into you for something else.

If it were a mutual breakup I'd say you two would have to talk and come to terms... But I'd liken this more to a cheating breakup (regardless as to whether something happened, she definitely did emotionally cheat on you) where the person doing the wrong thing doesn't deserve the talk, and the person being wronged shouldn't have to endure giving the talk. If you two talk she will probably throw out a bunch of babys and but i love yous and blah blah blah (you melt, stay together) and then 2-3 weeks later she will repeat her behavior.