Maybe I should elaborate on two of the options a bit:
About the time this started, I also beat up some guy who was annoying her in a bar. So, she might appreciate my macho-ness and think I'm the bomb because of that
Also, I took her to Paris last year for her birthday, and she's been hinting at wanting another birthday present like that. She might just be using this to try and get something like that again.
Ofcourse, she also went on vacation with a few of her friends, and I have no way of knowing what happened there, so cheating and feeling guilty might still be the reason...