B.G.
Are her ears bothering her..... maybe lying down flat is bothering a possible ear infection. Has she had a fever, tugged at her ears,?
Sometimes they need to be propped up while sleeping. It might make her feel better. My daiughter is 13 months and has to sleep on a pillow (a very flat pillow, but still....)
I dont mean to make you worry more, just could be a possiblity. I'm probably wrong, though.