A parturient woman is 27-year-old, it was her second labour, delivery was at term, normal course. On the 3rd day of postpartum period body temperature is 36,8°C, Ps - 72/min, AP - 120/80 mm Hg. Mammary glands are moderately swollen, nipples are clean. Abdomen is soft and painless. Fundus of uterus is 3 fingers below the umbilicus. Lochia are bloody, moderate. What is the most probable diagnosis?