Swollen eyes

Everytime I lay Ellie on her tummy to sleep when she wakes up in the morning her eyes are swollen and puffy. I've changed the sheets and I'm not sure why this happens any thoughts? I know
You will probably say just lay her on her back but she sleeps better on her tummy.

My sons eyes are always swollen in the morning
He had allergies

Ok because when she sleeps on her back she doesn't have them. Idk if I should continue to let her sleep on her tummy or not.