What Causes Swollen Face In A Kid While Sleeping At Night?

My 3 almost 4 year old daughter wakes up from nighttime sleeping and naps with a swollen face and looks a bit pale. She seems fine in every other way. It goes away it about a hour or so. What could be causing it? (She does have mild sleep apnea due to large tonsils and adnoids. She also may have seasonal allergies but not sure on that one)