Can a toddler shake them self to much to cause a brain bleed? my toddler rocks back and forth hitting his back hard on the couch repeatedly.

Unlikely: A baby can not causes any significant brain damage by their own movements. Now if they fall or hit their head on something, them the fall could cause damage. If a toddler is have a tantrum, for example, and they are kicking and screaming on a hard floor, they can cause at the extreme some local cuts or bruising but very unlikely to cause actual brain damage.