Was it something I did that drove her away?

Most likely, she never actually liked you. When she gave you her number, she was just flirting. You should have figured that out when she kept making excuses to not go out with you. She finally gave in because you kept begging her for a date and she felt bad about it. She probably did have fun on the date, but not enough to see you again. Whether the guy she's with is as good as you are is immaterial. He's the guy she chooses to be with and that's all that matters. When you catch her staring at you from afar, it's because she feels bad about what happened, not because she's interested in you. It was all a game, and you didn't play it very well; so stop being baffled and move on.