According to Miriam Stoppard, it is like bubbles bursting against your insides.
I felt a couple of bubbles yesterday but this morning when I woke, it was more pronounced. When I turned to my left, the bubbles were on my left side (head-butt). When I turned to my right, the bubbles burst on my right side (kick or knee-butt). Then there was the random bubbling at the top (punches). :D
Later, I told the hub about it and he came by to loiter some. I lay on my right side and he placed his hands under to hopefully feel a random kick. Turns out it was his own hand shaking under my weight. Suddenly BB kicked. I asked him, did you feel that? And he said yes! That's a kick! And he beamed proudly.