6 week old flailing/screaming?

Ch
Recently when i put my baby on her back she does this scary thing (at random times) where she stiffens her arms & legs (similar to the startle reflex) but then does these scream as of shes in pain & scared. She is fine the rest of the time, eats well, no fever, etc. this only started happening recently. Does anyone know what this is or has experienced this too?