I think this song, very basically means the following:.
Hayley (or the speaker) had feelings for the boy in question. However, she never acted on these feelings so the other girl (the supposed whore) and the boy didn't know she felt that way. Then the other girl began dating the boy, and even though Hayley (or the speaker) had no claim on him whatsoever, she was jealous of the other girl. The other girl and the boy have now broken up, Hayley (or the speaker) has begun a relationship with him, and is rubbing the other girl's face in it, so to speak. Very mature.