8 month old crying and acting scared when I set him on the floor

So I just put my 8 month old on the floor to play and practice crawling. I put him down in a sitting position on his butt, he can sit up on his own unsupported and he's usually fine playing like that. But when I put him down jjst now, he immediately started screaming crying and the look on his face looked like he was scared. He was actually shaking a little bit too. I picked him back up and he stopped crying. Now every time I set him down like that he screams and shakes and looks terrified. When I lay him down on his belly he is fine tho, and he rolls and scoots around like normal. It's just when I place him sitting up like that, he acts so scared. I've actually never seen him like that before. I am concerned by this. Does anyone know why he might be doing this?