Urinary Tract Infection (UTI): The patient's symptoms of epigastric pain only when urinating at night could be indicative of a UTI, particularly given the location of the pain and its association with urination. The fact that it occurs at night may suggest that the infection is causing irritation or spasms in the urinary tract that are exacerbated by the concentration of urine during the night.

Kidney Stones: Although the pain is described as epigastric, kidney stones can sometimes cause pain that radiates to the upper abdomen. The pain associated with kidney stones can be severe and is often worsened by urination, especially if the stone is moving down the ureter.
Overactive Bladder: This condition could potentially cause discomfort or pain in the abdominal area, especially if the patient experiences urgency or frequency at night, leading to nocturia (waking up to urinate).
Gastroesophageal Reflux Disease (GERD): While GERD typically causes heartburn and regurgitation, in some cases, it can cause epigastric pain. However, the direct correlation with urination is less clear unless the act of urinating somehow triggers or worsens reflux in this patient.

Pyelonephritis: An infection of the kidney, which can present with flank pain, fever, and other systemic symptoms, but could also cause epigastric pain if the infection involves the lower pole of the kidney or if there is significant inflammation.
Appendicitis: Although appendicitis typically presents with right lower quadrant pain, atypical presentations can occur, especially in obese patients where the appendix may be located higher in the abdomen, potentially causing epigastric pain.
Pancreatitis: Acute or chronic pancreatitis can cause severe epigastric pain that radiates to the back. While not directly related to urination, the timing of pain (at night) could be coincidental or related to dietary factors that also influence urinary habits.

Interstitial Cystitis: A condition causing bladder pressure, bladder pain, and sometimes pelvic pain. The symptoms can worsen with urination and could potentially cause referred pain to the epigastric region, although this would be less common.
Eosinophilic Cystitis: A rare condition characterized by an eosinophilic infiltration of the bladder wall, which can cause a range of urinary symptoms including pain. It's an unlikely cause but could be considered in the differential diagnosis of chronic, unexplained urinary symptoms.