In this case, where your hormones are still going nuts, exercise can be your best friend, even if you think you're too tired.
-It will energize you. It seems ironic, but even if you're tired, once you get moving, it helps you feel MORE energetic
-It helps you get happy. When exerted, the brain releases endorphins that literally have an anti-depressant effect.
-Calorie burn - I know this is obvious =)
*If you're nursing, just be sure not to deprive your body of much-needed fuel!
Try to avoid food / drinks that have caffeine as it will make you feel anxious, irritable and "on the edge". Make sure you're getting enough iron, folic acid, and B vitamins. It seriously makes a difference as depression is often linked to low levels of these things.
Please try to be patient with yourself as your hormones level out. Don't underestimate their ability to make you feel all sorts of crazy things. Keep reminding yourself this is temporary, take care of yourself and keep exercising (if your doc has given the ok, of course)
*hugs* and congrats on your new baby. It's ROUGH, no question, but you can do this!