A 31-year-old woman presents 12 days after a vaginal birth with increasing bleeding, fever and lower abdominal discomfort. Her temperature is 39.1°C, the uterus is larger and more tender than expected, and the lochia has an offensive odour. Transvaginal ultrasonography shows no convincing intrauterine mass and Doppler examination shows no focal vascular lesion. Which one of the following is the most likely diagnosis?