My belly is big enough now that when I’m sleeping, gravity pulls it down towards the mattress. The baby also gets pulled down. So, if I’m lying on my left side, my stomach and its occupant get shifted off to the left as well.
The other thing is that I have to sleep on my side now, and I have to switch sides periodically to stay comfortable. This is par for the course when you have the extra weight pressing on you from the inside. Anyways, I wake up every few hours with a numb hip, and roll over. When you’re pregnant, you can’t just roll over in your sleep, because it’s too much of a process.
When I roll and shift, the baby gets shifted along with me, and I get a lot of squirming. I imagine that it all seems very sudden to the kid, to have the whole world shift that way. I bet it feels something like an amusement park ride, but without your consent. In any case, it seems to take a lot of wiggling around to get comfortable again, for both me and the baby.