If MD says okay, then who am I to say otherwise?
Be careful ... like I said, if you are still bleeding and cramping, then I would wait. Your body is still recovering.
Also, know that working out can change the taste of your milk. Baby might not like it - lactic acid builds up in the milk too, altering the taste. You might want to make sure you time a workout around HIS feeding schedule. Breastfeeding can help you lose baby weight, but it also can keep that last layer of fat on you that is tougher to get off - you need a certain amount of fat to produce sufficient milk.
PLEASE don't stress about losing weight right now. It is going to take time and first priority is your son's health