We just got a pack n' play and both a mattress pad and sheet to go on it. When I had the mattress in the pack n' play by itself, it seemed perfectly fine -- the mattress covering was snug against the mattress. However, I then tried to put the mattress pad and sheet on the mattress (I'm just talking about that little folding one that the pack n' play comes with). While the mattress itself didn't fold up or anything, it seems like there is a lot of air trapped in it now -- the pad and sheet aren't loose at all, it's just like a big air pocket underneath. It seems like it's coming from the mattress itself, because now that I've taken the pad and sheet off the mattress, there's still some air in the mattress. Anyone else had this experience? Is it a concern? I want to be able to use a pad and sheet with the pack n' play, because the mattress itself doesn't seem waterproof, but having the baby sleep on an air pocket doesn't seem safe, either....