The cause of 'postpartum blues' is:
Puerperal Blues/3 Days Blues/Baby Blues

It is transient state of mental illness observed 4–5 days after delivery in nearly 50% of postpartum women.
Postpartum blues occurs at the height of hormonal changes. 
Patients present with depression, anxiety, fearfulness, insomnia, helplessness and negative feelings towards infant.
It may last from a few days to 2–3 weeks.
Generally self limited, 20% of women may develop depression in the first postpartum year.

Treatment: Reassurance and psychological support of family members.
Post-partum Depression

It is observed in 10-20% of mothers.
It is more gradual in onset over the first 4-6 months following delivery or abortion.
Changes in hypothalamo-pituitary-adrenal axis may be a cause.
Manifested by loss of energy and appetite, insomnia, social withdrawal, irritability and even suicidal attitude.
Risk of recurrence 50-100% in subsequent pregnancies.

Treatment: Should be started early. Fluoxetine or paroxetine is effective and has fewer side effects. According to kaplan, the cause of postpartum blues is :
“The sudden decrease in estrogen and progesterone immediately after delivery may also contribute to the disorder, but treatment with these hormones are not effective.”
Decreased estrogen
Decreased progesterone
Increased prolactin
Decreased estrogen and progesterone