21 month old shakes head when upset

My little boy shakes his head side to side sometimes when he’s stressed out/tired like after meals when he’s rushing wanting to get out his highchair so getting annoyed. It’s not all the time but it’s something he’s done since he was about 1 yo. We saw drs about it then and they said they thought it was behavioural and nothing medical and he could just grow out of it. He can go weeks without doing it then he’ll start up again but recently it seems a bit more mild if it does happen.

Does anyone else’s lo do anything like this? Ive read up on the possibility of autism but he’s met all his milestones and doesn’t show any other signs of anything like that? I just wonder if its just something he does that’ll fade out as he grows bigger cause like the drs said to us, kids can be strange! I do worry about it sometimes tho….