Body issues

It's been two weeks and four days since our little boy was born. I'm petite and he's a big boy born weighing 8/15. I was huge!
My issues are that I gained 25 lbs (after the water weight came off) and everyone says I look great, however my stomach and back muscles are still very weak. 
I want to lose the weight but also build back muscle.  
How soon did you start working out and what worked for you?
Thanks!!