That happens when your laying down. Ive had it happen to me too when i get the diaper used in the butt area. Sometimes having a too tight fitting diaper can cause it. Try going up a size.
I get tired and sleep more at night if I am wearing a nappy but comfort also helps. A few nights ago I have been using the tena ultima cloth backed nappies in a large but finding they are a bit big for me so I will have to try different medical ones next year. I also have kiddo teddy ultra in a large and these are great, I was wearing one yesterday with a onesie and had my paci in and I fell asleep within a few minutes. I put sent in my nappies also to make them smell like baby powder.