Post Partum Depression (Depressive Disorder occurring within 4 weeks of delivery) is a fairly common phenomenon, occurring in more than 10% of the females, more so after the first child birth. From your report, it seems the you are suffering from “it”. So now, there is no way out but to treat it.
Sertraline (Serlift) and Nortyptiline (Sensival) are good antidepressants and can be given in adequate dosage during pregnancy and breast feedings. So don't worry about the medications. These medicines have to be taken in adequate dosage (Sertraline - 200 - 300 mg / Sensival 75 - 150 mg) for at least 6 – 9 months, even when you are doing perfectly well.