A G1P1 woman immediately postpartum presents with severe vaginal bleeding. Her labor was remarkably quick for being primiparous, and her pregnancy was only complicated by fetal macrosomia. She has a history of leiomyoma. Along with severe bleeding, she has lower abdominal pain. The first one to arrive is a medical student who does a pelvic exam and says she feels a smooth, round mass protruding from the cervix. Her abdomen is flat. The patient quickly develops hemorrhagic shock.
Challenge: What's the diagnosis?