Your body is still trying to balance out your hormones from pregnancy so take it easy on yourself. And, your hormones will still fluctuate if you're nursing as well. Keep trying to get rest when you can...it does helps. Also, keep taking your prenatal vitamin which will help if your body is running low on iron and B vitamins, etc.
I have 4 children and have experienced both baby blues and postpartum depression. Based on my experience, I'd have to recommend a call or visit with your doctor. The baby blues usually only last a couple weeks after having the baby and anything more than that may be signs of postpartum depression. My OBGYN prescribed an anti-depressant called Zoloft which is safe to take while nursing. It has helped tremendously so that my lows don't feel soooo low. Being a mom and wife is a constant juggling act and it's obviously important for you (and most women) to feel good and capable about both roles. Don't be afraid to ask for help.
And, just for the record, even though I was diagnosed with postpartum depression...I NEVER felt like suicide or hurting my baby at any time. Nevertheless, the doctor will probably ask you if you have felt that way so you shouldn't take that offensively because they have to ask. I just knew that breaking out in tears and feeling so overwhelmed didn't meet my expectations while trying to enjoy my marriage and new baby.
What you are experiencing is normal but if you continue to not seem to be able to shake it off and be productive in your daily tasks then it might be time for intervention. And, like another mom had shared, you don't have to stay on medication long-term. It will get better and this too shall pass. Then, who knows, you might be asking yourself if you're ready to have a brother or sister for Duke. :)
It will all start to fall into place and as you get used to your new routines than it will become easier for you to not only get done what you need to but even enjoy life in the process. The fact is that sometimes postpartum depression does require medication and there isn't anything to be ashamed about. And, honestly, just "willing" everything to be okay doesn't always make it so. Don't be afraid to take action and also talk to your husband or trusted friend about how you feel. You know yourself better than anyone else so trust your instincts if you're still not feeling quite right.
Take care and enjoy the blessing of your new son!