A multiparous woman delivered by a village dai (midwife) presents on the 22nd postnatal day with bleeding per vaginam with clost. On examination, the uterus is 14-16 weeks, the internal os is open, and there is bleeding through the os. The likely cause of this bleeding is -
Excessive postnatal physical work Perineal tears Retained bits of placenta and membranes None of the above