There is a large wound left behind by the placenta after you have a baby (this goes for both vaginal and c-section deliveries), hence all the bleeding. Your body needs time to heal that wound, and therefore going overboard with exercise would not give you the time to do so. Also, your body needs to adjust all of your insides to get back into place and your uterus needs to shrink back down to normal size.
Walking is allowed, so maybe you can stick to that for now until you are cleared to work out. Also, of course, watching what you eat (but make sure you eat/drink enough if you are BFing, so that you don't affect your supply and are getting the nutrients you need).