It doesn't just dissappear. She's been thinking about it for a while. And while yeah, you didn't perform any of the cardinal sins (beating, cheating), I wouldn't pat yourself on the back for that. I thought so too at first, how could I be a bad guy if I didn't do those? It's like that Chris Rock skit, where he talks about how parents are like "Well, I feed my kids."
YOU ARE SUPPOSED TO FEED YOUR KIDS. Or in this case, you are supposed to not cheat, beat, lie, etc.
Most girls will let it accumulate. They think that by letting it slide, you'll eventually change for her liking, things will get better and you won't throw your love away. They don't want to seem bitchy, or needy, and they just hold it in. That's why break ups can get ugly, too much anger festering and lots of hurt feelings. Just like for you, it's a learning process they have to go through after being hurt enough times.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.