Pancreatic Pseudocyst: This is the most likely diagnosis given the patient's history of pancreatitis 6 weeks ago, current epigastric pain, and the ultrasound findings of a mass with a thick wall and high white blood cell count in the fluid. Pancreatic pseudocysts are fluid collections that can develop after an episode of pancreatitis, often presenting with abdominal pain and a palpable mass.

Pancreatic Abscess: This is another possible diagnosis, given the presence of a mass with high white blood cell counts, indicating infection. A pancreatic abscess is a collection of pus that can form in or around the pancreas, usually as a complication of pancreatitis or pancreatic surgery.
Walled Off Necrosis: This condition, also known as organized pancreatic necrosis, can occur after necrotizing pancreatitis. It presents as a well-defined, encapsulated area of necrotic tissue and can cause symptoms similar to those described, including abdominal pain and a palpable mass.

Pancreatic Cancer: Although less likely given the recent history of pancreatitis, pancreatic cancer must be considered, especially if there are risk factors or if the patient's symptoms and imaging findings do not fully align with the expected course of a pseudocyst or abscess. Missing a diagnosis of pancreatic cancer could have severe consequences.

Other rare conditions that could potentially present with similar symptoms and findings include intraductal papillary mucinous neoplasm (IPMN) complications, pancreatic lymphoma, or solid pseudopapillary neoplasm (SPN), although these are much less common and would be considered only after more likely diagnoses have been ruled out.