I can relate to her situation. If she asks you to go to a party, she probably wants to get you in a setting where she can try to figure out if/how much you like her back. That's always easier to do with a bit of liquid courage on board. I don't know much about this girl, but if she's a bit shy, like myself, asking you to come to a party may be a big deal for her. You then turning her down may feel like you're rejecting her, especially if you keep turning her down.
She might be confused because she likes you and she's picked up on some signals that you like her too (the look in your eyes, maybe you're a bit nervous around her, maybe you pay close attention to what she says etc.), and so she gathered the confidence to ask you to come along to a party, not just once but several times. If she's confused about your behaviour, it's likely she's annoyed with you for not being clear about what you want (trying to see things from her perspective here). However, if she likes you, she's not annoyed with you because she will hate you forever. If she is annoyed with you, it's because you have an effect on her emotions. You can pull those strings right back where you want them by doing one thing: Showing the decency and the courage to ask her out after you have, in her eyes, messed up.
This is just based on my own experience mind you. I'm pretty much in her situation right now and if the guy I like called to ask me out, I would definately say yes and hope for a later occasion to clear up any confusion.