Gastroparesis: This condition is characterized by delayed stomach emptying and can occur after gastric sleeve surgery. The symptoms of persistent nausea and vomiting, especially in the absence of other findings like esophagitis, ulcers, or gastric outlet obstruction (GOO) on EGD, make gastroparesis a plausible diagnosis. The normal EGD and CT abdomen without contrast do not rule out gastroparesis, as it is a functional disorder rather than a structural one.

Roux-en-Y Stenosis or Narrowing: Although the EGD was negative for GOO, a stenosis or narrowing at the gastrojejunostomy site could still cause symptoms of nausea and vomiting. This might not always be visible on a CT scan without contrast or might be missed on EGD if not specifically looked for.
Small Intestine Bacterial Overgrowth (SIBO): This condition can occur after gastric bypass surgery due to altered anatomy leading to bacterial overgrowth in the small intestine. Symptoms include bloating, abdominal pain, diarrhea, and nausea/vomiting.
Dumping Syndrome: Early or late dumping syndrome can occur after gastric sleeve surgery, characterized by symptoms shortly after eating (early) or several hours later (late), including nausea, vomiting, diarrhea, and abdominal cramps.

Internal Hernia: Although less common, internal hernias can occur after gastric sleeve surgery and may cause intermittent bowel obstruction, leading to nausea and vomiting. This diagnosis is critical to consider because it can lead to bowel ischemia if not promptly addressed.
Bowel Obstruction: Any form of bowel obstruction, whether due to adhesions, internal hernias, or kinking of the bowel, is a medical emergency. Symptoms can include severe abdominal pain, nausea, vomiting, and obstipation.
Peptic Ulcer Disease: Despite a recent negative EGD, peptic ulcer disease can still be a consideration, especially if the patient is using NSAIDs or has a history of Helicobacter pylori infection. Ulcers can cause nausea and vomiting, and their presence is a significant concern due to the risk of perforation.

Neurological Disorders: Conditions such as cyclic vomiting syndrome or even rare neurological disorders affecting the brain's vomiting centers could be considered, although they are less likely given the post-surgical context.
Inflammatory or Neoplastic Conditions: Rarely, conditions like Crohn's disease or a neoplasm in the gastrointestinal tract could cause persistent nausea and vomiting. However, these would be less likely given the normal EGD and CT abdomen without contrast.
Endocrine Disorders: Certain endocrine disorders, such as thyroid dysfunction or adrenal insufficiency, can cause nausea and vomiting. These are less directly related to the gastric sleeve surgery but should be considered in a comprehensive evaluation.