Hey, great job! The explanation is simple: you were in the zone! What's the "zone"? There's no answer, but it's more a state of mind than anything else. When in the "zone", you have a sense of confidence that whatever you do is going to work out OK at the end. That's what you were feeling when you met that girl at the bar. You probably went there just to have something to do, and talking to her was just another way to pass time, rather than "working". You probably didn't care how it'll turn out at the end; talking to a cute girl was more fun than just sitting there, so that's all you were concerned about. Hence, the zone.
The only advice I can give you is to forget about the three-day rule. Given people's short attention spans nowadays, it's very outdated. Try to find any way you know to get into the same zone you were in on Friday night, and call the girl. Don't spend too much time talking (my guideline is between 5 and 20 minutes), and set up another date with her. Just make it somewhere other than at a bar where you met, preferably something quieter and more laid-back, like a nice restaurant or a movie theater. If the attraction between you two is strong enough, you can make out during the movie. Choose something without intense action to facilitate the process .