DS is 18 weeks old and rolling is his current obsession. He’s fab at rolling from back to tummy, but when he goes tummy to back he often goes straight from a tummy time position, propped up on his elbows, to smacking the mat with the side or back of his head as he rolls. He’s on a play mat and he never seems upset but it looks a bit alarming. Is this OK/normal? Should I be hovering and trying to stick my hand under his head to catch him??