My 18 month old son is going through a stage of hitting me if he is told 'no.'

I have tried to keep my telling off simple by saying 'stop' or 'no' in a stern voice so he knows he has done something wrong but this doesn't seem to work as he will hit out several times. He definitely knows what the word 'no' means!!

He also definitely knows he has done something wrong as when my husband tells him off for doing it, he will shy away and look sad.

Is it just a phase?? I am so embarrassed when i am out in public and he does it, he is so strong and i feel i can't control him at the moment!!

Any advice or reassurance will be helpful!