My daughter has a hard time sitting & moves alot.What could it be frm?

Since my daughter was able to sit in a high chair she has been unable to sit still. She will sit in the chair have her arms in front of her and move back and forth. I don't know why she does this. The doctor told us that it could be a behavior that she picked up and for us to correct her on it, but every time we tell her to stop she will do so for a few seconds and then will start moving again. My mom noticed the other day that my daughter did this when she sat her down to eat as she usually does and noticed that my daughter had a frightened look on her face. She asked my daughter what was wrong and my daughter stated that it was her fingers. My mom said that her fingers looked like they had locked up on her and my mom massaged them. This is the first time that has happened to her. Does anyone know where this comes from or why it happens.