Does this usually happen?

This kind of thing has happened to me too. I guess it happens to a lot of people because there was a study about it that was reported in the New York TImes a couple of months ago.

I forget the details of the study, but the main point was that people who have just broken up/gotten divorced generally find another partner more quickly than the average single person. The study authors think that this happens because people think that someone who already has a partner must be good because they were chosen by their partner!

I guess this phenomenon is the reason that so many people try to make the people they want jealous by flaunting themselves with someone else!

My advice is that there must have been a reason that you didn't want this guy to start with. Even though it's nice to have admirers, it's human nature to get jealous when an admirer finds a partner. Laugh at yourself, leave the poor guy in peace to enjoy his new relationship, and make sure you're around when he is free again if you still feel the same way.