Congratulations on the baby! What a cutie.
You're going through a lot of changes at once and that can really affect your mood. It's not necessarily postpartum depression, but something you should mention to your doctor.
As far as workouts, I walked a lot in the initial stages with each of my babies. I had 3 emergency c-sections, and that was all I could do. I believe I started at about 4 weeks. It worked well--I was on maternity leave and could put the baby in a stroller and walk. Start easy, and back off if it doesn't feel good. The exercise will help your body and mind, so do what you can.