Some things to check out: food sensitivities can cause these types of reactions. I saw one happen in real time in a child once, who went from a well behaved, rational 8 year old, to whining, hitting his mother and unable to sit still or read or do any of his normal activities after eating some candy - turns out he had a sugar sensitivity.
Also check to see if your child is autistic.
If you live near Salem, MA there is a very good clinical psychologist there who can diagnose if there are family issues, school issues, or some brain malfunction going on. His name is Dr. Richard LeBel at ###-###-####