Hi there. Has your child ever had a complete multidisciplinary evaluation (MDE?)
I would recommend this to get the most extensive sort of testing for a child of her age. It may be impossible to know what is going on when she is so young, but at least a complete evaluation can point out the child's problem areas and address them. Also, check your family tree for psychiatric/neurological disorders or substance abuse. Often things are hereditary.
Kids who act out usually have some sort of disorder fueling the behavior. I would check not rely on a pediatrician or plain therapist or psychologist to check it out--in my opinion, they don't test enough.