A 28-year-old male with a 5-year history of duodenal bulb ulcers, recurrent melena, and intermittent proton pump inhibitor (PPI) treatment; 4 hours ago, he experienced hematemesis 3 times (total ~600ml) and black tarry stool (melena) 3 times (total ~200g), accompanied by marked dizziness, sweating, palpitations, and multiple episodes of syncope upon changing from lying to sitting position. The most likely diagnosis is: