What is it specifically that moves around and makes clothes fit differently?

How discouraging to bust your butt getting down to pre-preg weight and then clothes don't fit.

I wore my jeans and stuff pretty tight before I was pregnant. And I love a nice designer jean. I somehow HAVE to get back into them because they were all too expensive! I will be so mad if I get back down to my dream weight and I still can't wear them!