I also have an 11 year old daughter who has mood swings, but has not started her monthly cycle yet.
I've learned that as long as you aren't taking your child to the doctor for every sniffle and cough, that it is fine to take her when you really feel something isn't quite right. It never hurts to get a professional opinion. I struggle with the decision to take her in to the doctor all the time, figuring they will say it's nothing or a virus that has to run it's course, but worrying that it could very well be something more serious.
Better safe than sorry I always say. With so much going on in today's society, depression is so much more prevalent and not something to shrug off. If she is experiencing an imbalance of some kind, it's better to start taking care of it earlier than later.
Good luck