A puerperant is 28 years old. It's the 3rd day post-partum after a second, normal, term delivery. The body temperature is of 36,8°C, Ps- 72/min, AP- 120/80 mm Hg. Mammary glands are moderately engorged, the nipples are clean. Abdomen is soft, painless. The fundus is 3 fingers' breadth below the navel. Moderate bloody lochia are present. What diagnosis can be made?