Nonsteroidal Anti-Inflammatory Drug (NSAID)-induced gastritis or peptic ulcer disease: This is a common complication in patients with rheumatoid arthritis (RA) who are frequently treated with NSAIDs, which can cause stomach irritation, leading to gastritis or peptic ulcers. The symptoms of acute epigastric pain, nausea, and vomiting, along with elevated liver function tests (LFTs) and bilirubin, could be related to NSAID use, especially if the patient has been taking these medications for an extended period.

Gastroesophageal reflux disease (GERD): Although less directly linked to the liver function abnormalities, GERD can cause epigastric pain and nausea. In patients with RA, the use of certain medications or the disease process itself might exacerbate GERD symptoms.
Medication-induced liver injury: Certain medications used in the treatment of RA, such as methotrexate, can cause liver injury, leading to elevated LFTs and bilirubin. The acute epigastric pain could be a nonspecific symptom associated with liver dysfunction.
Infectious gastroenteritis: This could cause acute onset of nausea, vomiting, and epigastric pain. However, the elevation in LFTs and bilirubin might not be as directly explained unless the infection involves the liver, such as hepatitis A.

Peptic ulcer disease with perforation or bleeding: Although this might seem like an extension of the most likely diagnosis, it's critical to identify and treat promptly due to the high risk of mortality associated with perforation or significant bleeding.
Acute pancreatitis: This condition can present with acute epigastric pain, nausea, and vomiting. Elevated LFTs can sometimes be seen in pancreatitis, especially if there's associated liver injury or bile duct obstruction.
Hepatic or splenic infarction: These are rare but potentially life-threatening conditions that could present with acute abdominal pain and elevated LFTs. They might be more likely in patients with certain underlying conditions, such as antiphospholipid syndrome, which can be associated with RA.

Autoimmune hepatitis: Although rare, this condition could present with elevated LFTs and bilirubin. It might be considered in the context of RA, given the autoimmune nature of both diseases.
Primary biliary cholangitis (PBC): This autoimmune liver disease primarily affects the bile ducts and could present with elevated LFTs. However, it's less likely to cause acute epigastric pain and nausea as primary symptoms.
Hepatobiliary involvement in RA: Rarely, RA can involve the liver or biliary system directly, leading to symptoms and laboratory findings similar to those described. However, this would be an uncommon presentation.