Either way, your problem is most likely solved. If she claims to want to stick around and then starts mewling again, just heave a deep, sorrowful sigh every time she acts up. Brush a tear from your eye if you can do so in a faux-surreptitious manner that's not too forced. After a few rounds of this she'll realize that she's wrecking your festive mood and will learn to shut her big mouth, if she is indeed your friend. Friends are generally racked with guilt over such things.
Bridal attendants freak out in droves.
Common reasons include feeling envious of your committed relationship or being afraid of losing you as a friend once you're married. Maybe if you act concerned and understanding enough, she'll let you know about what's really gotten under her skin. One thing you absolutely can't do is blow a spaz at her, no matter how tempting or justified such a spaz might be. Otherwise, word will get around that planning your wedding has turned you into a hideously wicked princess with no regard for the feelings of others. People have a way of remembering when and how others have been mean to them, long after the memory of their provoking that meanness has slipped their minds entirely.