What Causes Violent Behavior In A Child?

my son is 23 months and very voilent, cranky and cries alot aswellas VERY sensitive. daycare says he will not sit for longer than 2 minutes. he will perposely hit his head off walls the floor anything. by voilent i mean hitting nspitting kicking bitting(even himself til he draws blood). what can i do.? please help me. is it a sign of add/adhd or autism?