¿What?

[Saxophone 44]

I was talking to a girl a couple days ago, and she was just laying on the frosting pound-by-pound. We have quite a Past, this girl and I, and she's never come onto me like this before. Even when she asked me out. I started saying the same stuff about her, because I have a little thing for her. But the next day, she starts talking about this other dude. She is now dating that other guy. This is where the title comes in: ¿What?

[FullyAlive]

It sounds like she was playing a game. She seems to have wanted you to ask her out or something of the sort, you didn't so she got someone else instead as some form of 'fuck you'
That's purely a guess but that's what it sounds like, I think you should just walk away she's not mature enough to handle anything anyway.