Well, its always possible that she has ADHD. Basically it means that besides being hyper, the younger child has no filters. They will do something with out thinking. As they grow older (middle school or higher), they are able to think twice. I have seen many kids that literally don't realize they have done something because they acted on the impulse so fast, and then moved on to other things and that point in time was lost to them.
What this means is that if it is true that she has ADHD, then the conventional means of discipline won't work. Spanking her is a huge waste of time, and teaches her the wrong way to handle situations. Get a book like, "The ADD/ ADhD Answer book." , by Susan Ashley and follow it. The book will also help you decide if she has ADHD - although she could just be a very smart manipulative kid. At that age, you probably won't get many doctors to diagnose her condition. I would ask her teacher what she thinks. She has been around kids long enough to have a good opinion. If its not ADHD, then I would recommend getting, SOS Help for Parents by Lynn Clark. Hope this helps.