So this is new, my 18 month old has started hitting regularly. Sometimes it's when he's annoyed at something, other times it's out of excitement. He's never been hit a day in his life by anyone so I'm really baffled as to where this has come from?...I sternly tell him 'no' and turn away from him or put him down to show I'm upset, but 5 mins later he will do the same thing again. Is this normal?