As a girl who I am sure has done this to guys before, I can tell you what I think. From all that you said, my opinion would be she just wants to keep you damgling. I mean, girls like to be desired. We want to be wanted. We like the thought that guys are out there thinking about us. It's a control thing maybe, like having the upper hand or something. She dated you so she knows you like her and would want to be with her. Then she had to go home. So maybe something better came up, maybe it didn't, maybe she just wants to keep her options open. So she emails you casually and invites you to her brother's party. This is to see how bad you want her. If you jump on the opportunity to see her and rush up to the party, she wins. She knows that she still has the upper hand and that you want her bad. You didn't go, but emailed her back. Shows you're not a dick and shows you care on some level. Email exchange takes place and you agree to see her. This tells her that you want her. This is as good as having the upper hand again. She doesn't have to see you because she knows that she could and that's what matters. She's been doing her things and then, out of courtesy, emails you to tell you she's been busy. This keeps her from looking like a bitch thereby making you not want her and thereby taking away her upper hand. Bottom line, she's yanking you around so she knows that she always has you available if she wants you. If she cared about you, she would have made more of an effort. No body gets THAT busy in the summer time. I mean, if she has obligations, she still could have emailed you and invivted you to come see her or something. She could have made more of an effort. She's just pulling ypou along as a back up. I mean, if she's lonely one night, she always knows she can call you. But if something better or more permanent comes along, she'll forget about you. Forget about her, you don't need her. You can do better. Girls are bitches! Don't let her walk all over you. Find someone that treats you better.
Maybe I'm wrong. Maybe she really has been busy. Maybe there's more to the story that I don't know or maybe I'm misinterpreting thigns. But, from what I read in your post, that's my opinion, for whatever it's worth.
1212 Out.