Baby kinda choking herself in the sleep sack

Zaza

Basically she’s wearing a sleep sack, but loves kicking her legs in the air. As a result, her feet are pushing against the top of the sack, and pulling quite harshly from the shoulders. I’ve seen marks on her shoulders as a result of this. What do I do?

She’s 5 months old and I’m using a 6-12month dreamland sack