A 40-year-old woman with known Crohn's disease, currently in remission on azathioprine, presents with a 3-day history of gradually worsening central abdominal pain, vomiting, and constipation. She has not opened her bowels or passed flatus for 24 hours. On examination, her abdomen is distended and tympanitic, with high-pitched bowel sounds. An abdominal X-ray shows dilated loops of small bowel with multiple air-fluid levels. What is the most likely diagnosis?