What does it mean if your baby keeps pushing his back off the bed with his legs?

probably gassy. or has a tummy ache

@steph11, but he has a smile when he is doing it, then when he is in his bobby pillow, he will do it to slide out of it?

then he might be trying to strengthen his legs to roll and such. every baby is different. if he's not fussing I wouldn't think he's gassy.