Baby moving sideways in crib?

Hi! I have been finding my 3 month old sideways in her crib when I pich her up for a night feeding.

i put her to bed with her head to the top and feet to the bottom of the crib. And I find her with her head to one side and feet to the other side.

we recently upgraded her from a bassinet to a crib. The mattress is firm, only with a fitted sheet, no bumpers or anything else in the crib. She does not sleep swaddled and wears a sleeping sack. She has not learned how to roll yet.