I know exactly the feeling you're talking about. This is my second pregnancy, and probably because of that, I can see it too. When I feel stretched, I'm huge! And then sometimes I'll wake up in the morning and be like "Where'd my baby go?!" Sometimes, usually later in the evening, the babe changes position to sort of lean out, so that the big lump I feel is the curve of her spine and buttocks. Or she'll shift around so that a foot or knee is kind of permanently trying to poke out on one side of my bellybutton (in cases like this, we can play tag together, since she can feel my light counterpressure as well).
Anyway, my.02 is that it sounds like changes in fetal position - you have an active little swimmer on your hands! :-)
mama to Samuel 08/02 and due 01/17/03