A 35 year old patient presents with a four day history of vaginal bleeding. Ten days ago she had a forceps delivery for delayed labour at 39 weeks in her second pregnancy. On examination her pulse and blood pressure are normal. The uterus is enlarged and tender. What is the most likely cause of the patient's symptoms?
1) Endometriosis
2) Uterine fibroids
3) Placenta previa
4) Postpartum hemorrhage