A 30-year-old, G1P1001, previously healthy woman presents 4 weeks after her cesarean section for a postpartum examination. Her labor was complicated by fetal bradycardia requiring emergent cesarean section. The patient complains of difficulty breastfeeding, for which she feels quite guilty. She also complains of fatigue, which she attributes to not eating much due to loss of appetite. Because of her fatigue, she spends most of her time sleeping and reports that she is not really interested in going out or doing activities at home since her delivery. She denies vaginal bleeding and abdominal pain. Her vital signs are within normal limits. What is the most likely diagnosis?