Abdominal swelling at night?

I'm a around 17 weeks and last week I noticed when I first lay down for bed on my back and suck in and feel my stomach if feels relatively normal as if I'm not pregnant. However throughout the night after lasting on my sides I'll turn on my back and suck in and feel my stomach and it's pretty firm and more swollen as if I actually have q baby bump . I know it sounds crazy but it felt kind of crazy lol anyone ever feel this?