Rectus sheath hematoma is an uncommon and often misdiagnosed cause of acute abdominal pain. We report a patient with rapid-onset hemodynamically unstable rectus sheath hematoma in conjunction with receiving anticoagulation therapy. A 54-year-old woman was admitted to the emergency department, complaining with a 4-hour history of sudden severe right upper abdominal pain. She had been using warfarin and diltiazem. There was no finding at ultrasonography in admission. At the second hour of follow-up, re-examination revealed an elongated tender mass in the right abdomen and little bluish discoloration at the same area, and Hb and Hct levels had dropped and the patient become hemodynamically unstable. Abdominal computed tomography showed the mass correlated with anatomic position of the rectus sheath. Rectus sheath hematoma may occur more frequently than previously described in relatively elderly patients. The classic signs of localized tenderness or mass with a characteristic history of straining may be completely absent. The diagnosis can be made confidently by urgent computed tomography scanning especially in patients without mass; ultrasonography may miss the diagnosis in early stages.