Sounds like you both got over one another pretty quickly, though I'd hazard a guess that neither of you considered your relationship to be serious, if you're dating again after only three days. Better be prepared for more encounters if you live in a small town. Same goes for new guy, if that doesn't last, as well.

although he could have been out with friends while she was the only one of the two there on a date.

True. He could have been there trying to get over her, and her turning up made him feel worse. Especially since she'd moved on already. If that was the case, this is more his FML than OP's. If they were both there on dates, they should chalk it up as one of countless awkward moments life will throw at them, and move on.

Good thing you had a spare boyfriend to play with.