For some reason your story is giving me deja vu (spelling?)...I've heard this story before but from the mom of the hyper child's best friend. Is it possible that your child acts up frequently? You mentioned that her best friend has been switched to a different after school class...this was mentioned in the other post by the other child's mom.
I think you need to discuss everything with the other child's mom. If it is the same person who posted earlier then she feels terrible about this. She felt like it was difficult to approach you and was torn up about it (for you, her daughter and herself).
Even if this is a different set of people I would speak to the other mom and be open minded. Maybe your daughter acts out too frequently and it has become an issue for others. It's not easy to admit (or see) that there may be a behavioral issue with your daughter but as a mom you owe it to her to find out and take care of her the best you can. I'd speak with your pediatrician after speaking with the other mom so you can explain details of what your daughter does when playing with others.
Also...you mentioned that the party was the first time you left your daughter alone for 2 hours and that you would never do that again. This makes me think that you are aware of behavioral issues. I have a daughter (11) and a son (8) and they play for entire days with other kids and I never am worried about their behavior. Over the years I've drummed it into their heads to play fair, include everyone, etc. Also, my son gets very hyper after he's had sugar but he has never had a problem with playing with others. The parents always compliment me on my child's manners and good behavior. I'm not bragging but I want you to know that I work on my kids' social skills every day and have been doing so for years. I never let them slack. In fact, I'm known as the mom that wants to know if their kid misbehaves. I can't help my kids fix their behavior if I'm not aware of it.
My kids aren't perfect (by any stretch), I've been called a couple of times about them hurting other kids feelings but I came down on them so fast that they really understand when they've done something wrong.
Please don't think I'm preaching...I just feel that it's our responsibility to make sure our kids no right from wrong in all settings...possibly most importantly when interacting with others. This is a life long lesson and now is the time to form the proper behaviors. They learn from us. It may be embarassing and difficult for us to hear when our child isn't acting appropriately but it's better that we hear it and work on it then ignore it.
Hope this helps and good luck!
L.