DS is 18MO this month and recently has started banging his head on the floor when being told off.
"Telling off" is usually telling him gently "no" when he's throwing his toys around/hitting his sister etc. He seems to hit people in frustration too and this is definitely not learned behavior.
He bends over from a standing position and bangs his head on the floor once or twice before continuing on with what he was doing, but he's done it hard enough to leave a bruise 
Strongly considering getting him a helmet! Any ideas on why he's doing this and how to stop it would be appreciated.