moving about in sleep

[Deardinah]

Up until three weeks ago 18m/o DD had slept in a sleepyhead mattress and so remained in the same place in her bed. Since moving up to the toddler room at nursery we have taken the mattress away and had her sleeping on the normal mattress of her cotbed. She's sleeping in a toddler bed at nursery, so i took the opportunity to remove the sleepyhead. She sleeps well as a rule, but lately has started to move about a lot during the night and started to hit her head quite hard against the top/bottom/sides of the cot. (we can hear it from downstairs!) It sometimes disturbs her sleep but mostly she'll just murmer and go back to sleep, i had to take the cot bumper away as she was getting herself all wrapped up into it!
Does anyone have any advice? She sleeps in a sleeping bag, i don't use any toddler bedding, but wasn't sure if i introduced a pillow, it might help?

Ds did this alot, found out that he actually sat up, still asleep and dropped back down! But the banging actually sounds worse than what it is, our reasoning was if it hurt they would defiantly wake up. We gave him a pillow around 2 yeats, but to be honest he just out grew moving around so much while asleep

My ds also does this ge regularly does laps of his cot moving around and sitting up in his sleep then slumping down sometimes hunched over looking really uncomfortable. I've had to learn to just leave him to it as I was waking him up by trying to lie him back flat on his back