The whole plot of Chasing Amy is the unfolding events of how a girl becomes this to the protagonist. The title of the movie itself is presented as another way of saying "The One That Got Away", and this trope could in fact be renamed to that with little difference in meaning.
Silent Bob: So there's me and Amy, and we're all inseparable, right? Just big time in love. And then four months down the road, the idiot gear kicks in, and I ask about the ex-boyfriend. Which, as we all know, is a really dumb move. But you know how it is: you don't wanna know, but you just have to, right? Stupid guy bullshit. So, anyway, she starts telling me about him... how they fell in love, and how they went out for a couple of years, and how they lived together, her mother likes me better, blah, blah, blah, blah, blah... and I'm okay. But then she drops the bomb on me, and the bomb is this: it seems that a couple of times, while they were going out, he brought some people to bed with them. Menage à trois, I believe it's called. Now this just blows my mind, right? I mean, I am not used to this sort of thing. I mean, I was raised Catholic, for God's sake.
Jay: Saint Shithead.
Silent Bob: So I'm totally weirded out by this, right? And then I just start blasting her. Like... I don't know how to deal with what I'm feeling, so I figure the best way is by calling her a slut, right? And tell her she was used. I'm... I'm out for blood. I really wanna hurt this girl. I'm like, "What the fuck is your problem?", right? And she's just all calmly trying to tell me, like, it was that time and it was that place and she doesn't think she should apologize because she doesn't feel that she's done anything wrong. I'm like, "Oh, really?" That's when I look her straight in the eye, I tell her it's over. I walk.
Jay: Fuckin' A!
: No, idiot. It was a mistake. I didn't hate her. I wasn't disgusted with her. I was afraid. At that moment, I felt small, like... like I'd lacked experience, like I'd never be on her level, like I'd never be enough for her or something like that, you know what I'm saying? But, what I did not get, she didn't care. She wasn't looking for that guy anymore. She was... she was looking for me, for the Bob. But, uh, by the time I figure this all out, it was too late, man. She moved on, and all I had to show for it was some foolish pride, which then gave way to regret. She was the girl, I know that now. But I pushed her away. So I've spent every day since then, Chasing Amy
... so to speak.