Small Bowel Obstruction: The patient's symptoms of worsening abdominal pain, nausea, vomiting, and decreased bowel movements, along with the abdominal X-ray showing a small air-fluid level in the left upper quadrant and paucity of colonic bowel gas, are highly suggestive of a small bowel obstruction. The presence of a known stricture, which is a common complication of Crohn's disease, further supports this diagnosis.

Crohn's Disease Flare: The patient's symptoms could also be indicative of a flare of her underlying Crohn's disease, particularly given her history of a stricture. However, the presence of significant abdominal pain, nausea, and vomiting, along with the radiographic findings, makes small bowel obstruction a more likely diagnosis.
Gastroenteritis: Although less likely given the patient's underlying condition and the specific radiographic findings, gastroenteritis could cause similar symptoms of nausea, vomiting, and abdominal pain.

Bowel Perforation: Although the abdominal X-ray does not show evidence of bowel perforation, this is a potentially life-threatening complication that must be considered, especially in a patient with a history of Crohn's disease and stricture. Clinical deterioration or signs of peritonitis would necessitate urgent reimaging or surgical consultation.
Toxic Megacolon: This is a serious complication of inflammatory bowel disease that can present with abdominal pain, nausea, vomiting, and decreased bowel movements. The absence of significant colonic dilation on the X-ray makes this less likely, but it remains a critical diagnosis not to miss due to its high morbidity and mortality.

Intussusception: Although rare in adults, intussusception can occur, especially in the context of a lead point such as a stricture or tumor. The patient's symptoms and the presence of a small air-fluid level could potentially be consistent with this diagnosis, but it would be less common than small bowel obstruction or a Crohn's flare.
Mesenteric Ischemia: This condition can present with severe abdominal pain out of proportion to physical examination findings and can be associated with nausea and vomiting. However, the patient's symptoms and radiographic findings do not strongly suggest this diagnosis, making it less likely but not impossible, especially in the absence of other clear risk factors.