Baby’s eye swelling

My little one, the past couple of times that she’s spent an extended period of time sleeping on the left side of her face, has woken up with a swollen left eye. No crusties or redness or anything. It’s just swollen. After a diaper change and bottle feed it starts to open up. She usually favors her right side and will sleep on that almost all through the night and not wake up with any eye issues. She’s two months and has favored that side of since birth with no issue. Is this happening because she’s just started changing the way she sleeps? I noticed it first after she fell asleep on her tummy facing her left side for over an hour. Now this morning after spending the majority of the night sleeping on her left side it happened again, but it’s already started to open up. What could be causing this?