I'd have to say it's probably jealousy. You let her meet your new girl, she sees how well you are treating the other girl and gets pissed. Nevermind the fact that you laughed together the enitre night - women are crazy like that. They can want to kill each other but still act like best friends. And it doesn't necessarily mean she still has feelings for you (although that is a possibility), she just sees that you are giving someone else the love/attention that was once hers and she hate it.
