I agree with samspade but you haven't provided enough detail in your specific situation. Is this a date you organized with her, for the primary purpose of seeing her, or is this an event you both happen to be going to? It sounds like the latter and if so, you have zero grounds to be upset or see it as a bad sign on her part - just hang out and have fun and make the best of the situation.
But why have you been "talking some" with a girl without having asked her on a DATE? To me that's part of the issue right there. That would prevent situations like this. This makes it sound like you are hovering around her being all "nice" in hopes she gives you a clear signal that it is safe to proceed, in order to eliminate any chance for rejection on your side. That's not a good way to go.
Also, hearing who she's going there with from a mutual friend seems strange to me as well..... Why are you talking about her to mutual friends? This seems like you are being really shy and weak... Asking about her, telling people what you're doing together etc, which is really bad for a number of reasons. It will get back to her which will make you look extra weak. Especially since you haven't even been on a date with this girl! Why so much thought and investment in someone you haven't dated, slept with, or even kissed?
1. See girl you like
2. Ask girl out on DATE, 1:1
3. Anything other than a yes, and you can avoid all this kind of BS and put her in your past
4. Don't talk about her to mutual friends!