Same thing happened to me once. I was 2 or 3 weeks with the same girl but i actually liked another girl from the same bar. I could not get her of course because i was stuck with the first one. But suddenly girl no.1`s regular boyfriend came to town and she actually said to me: you can go with her now (the girl i wanted). Yesss, finally i thought! So she was setting me up with this girl but after a while sitting in the bar (Me with the new girl, and she with her regular boyfriend) she seemed to have changed her mind and started to look really grumpy. So i barfined my new girl and took her to another bar. Then girl no.1 started to call me, but i did not answer. After a while she managed to track us down in the new bar and made a scene, i ended up empty handed of course
I guess they dont always know what they want, especially if they are drunk.