My son does that all yhe time, hes head down and his butt/body is on my left side and his legs are on the right, its a crazy feeling when he wants stretch his legs out lol.
Idk what my son was doing but its like he would put both hands on one side of my belly and then both feet on the other side and just spaz out. I mean it was crazy and he did that a few times. So its possible your baby is just stretched out and feels like knocking on one side a little more than the other(:
Yeah, for me my boy likes to kick and push his body out at the same time making my stomach really lopsided lol it's easier to tell what's going on when you know the position of the baby