No. But I don't know how to stop it. I think it's the same as being a good loser in sports. No one likes losing. There are a lot of jerks out there that curse and scream and offer excuses when they lose a game. The good losers may feel like doing the same thing. But because they're cool, they put on a smile and compliment the winner.
You still probably have some feelings for your ex-boyfriend. It's tough to see him dating someone else. But if he's a good guy and your best friend is a good person, then it may be righteous that they get together for at least awhile.
Caring about friends' jealousies is certainly one reason why I'm VERY careful about dating friends' ex-girlfriends. However, try to put on a smile and be as cool as you can about it if this happens.