My Girl Is Dating Another While I Was Not Around. What Should I Do

If you want it to be an exclusive relationship give her an ultimatum.  tell her that you want to be exclusive or just friends.  if it was already exclusive then you should probably just break up. if she really cared about you she wouldn't be hurting you like that.

lots of luck

Walk away & find another girl friend . If shes done it once she my do it again . Don't let yourself get hurt.  walking away is a good thing you may find the grass is greener on the other side of life.

Ultimately it is going to be up to you, but, if she is going to be unfaithful for even a short period of time, then there isn't really a sure thing in this relationship as she will most likely do this again.
Fool me once, shame on you, fool me twice, shame on me. Does this help? I hope so. Good luck to you.

If she was doing that behind your back that obviously shows that she isn't really commited to the relationship. You deserve someone better

Dump her. She is not showing you any respect or honesty.

You need to move one!! There will never be any trust there!!! Honesty is the best way to go!!! Best of luck to you!!!! You will find the right person and one that will treat you right!!!