I am also a mother of three, but mine are big kids and the "baby flab" is pretty well established!
I have been fit, and have "started" back many times. I know that if I am so sore that I can't move, I need to back off a little. Actually, the second day post strenuous workout is usually the worst for me.
Stretching or doing some light exercise, like just walking, seems to help me with the soreness. Today is my day two and I am sore also...Grateful for the things that keep me moving today!