M.
Sounds like you have a diastasis, also known as a "mummy tummy" and be forewarned, it can be exacerbated by doing the wrong type of ab exercises (like the pilates 100 - bad for a diastasis, typical sit ups are also not good!). You need to get your transverse muscle engaged and strengthened before you can effectively work all of your other ab/core muscles. Here's an abbreviated explanation of the diastasis and some exercises you'll need to do to make your diastasis smaller (reduce the tummy):
http://www.maternalfitness.com/education/lose_tummy_semin...
http://www.maternalfitness.com/baks_basics.pdf
I'm still working on my diastasis, and it's now down to 2 fingers, but I've been slacking lately and think if I put any sort of regular effort into doing my exercises, I could probably get it down to a 1 finger's width separation. I know it sucks, though, being asked if you're pregnant when the baby came out months before - happened to me too!