Metastasis to the liver or peritoneal cavity: Given the patient's history of stage 4 lymphoma, it is highly likely that the abdominal pain could be due to metastasis to the liver or spread to the peritoneal cavity, which could cause tenderness on palpation.

Constipation: Especially in patients who are on pain medications or have reduced mobility, constipation can cause significant abdominal pain and tenderness.
Gastrointestinal obstruction: Although less likely without a history of recent abdominal surgery or other predisposing factors, a partial obstruction could cause pain, especially if the patient has had previous abdominal surgeries like an appendectomy.
Infection: Given the presence of a pleural drain, there's a risk of infection either at the site of the drain or more systemically, which could cause abdominal pain among other symptoms.

Bowel perforation: Although rare, especially without a clear precipitating event, a bowel perforation is a medical emergency that requires immediate attention. The patient's history of lymphoma and potential for metastasis or treatment-related complications increases the risk.
Pulmonary embolism with referred pain: While the primary complaint is abdominal pain, in a patient with a pleural effusion and malignancy, pulmonary embolism is a significant concern that could potentially refer pain to the abdomen.
Spontaneous bacterial peritonitis (SBP): In patients with cirrhosis or significant ascites, SBP is a concern, but it could also be considered in patients with malignancies affecting the liver or peritoneal space.

Hepatic vein thrombosis: This condition, also known as Budd-Chiari syndrome, is rare but could be considered in patients with malignancies that increase the risk of thrombosis.
Primary or metastatic tumors of the abdominal wall: Although rare, given the patient's history of lymphoma, it's conceivable that the pain could be due to a tumor of the abdominal wall.

Imaging: Ultrasound or CT scan of the abdomen to evaluate for liver metastasis, peritoneal spread, or other abdominal pathologies.
Blood work: Complete blood count (CBC), liver function tests (LFTs), and coagulation studies to assess for infection, liver dysfunction, or coagulopathy.
Pleural fluid analysis: If not recently done, to assess for signs of infection or malignancy in the pleural space.
Abdominal X-ray: To rule out bowel obstruction or free air under the diaphragm suggestive of perforation.