Spontaneous Hemorrhage: Given the sudden onset of subxyphoid pain and the presence of free fluid in the abdomen, a spontaneous hemorrhage, possibly due to a ruptured hepatic or splenic artery aneurysm, or even a bleeding ulcer, could be considered the most likely diagnosis. The absence of significant findings on CBC (complete blood count) and normal lipase levels might initially seem to rule out some common causes, but in the context of sudden onset and specific location of pain, hemorrhage becomes a prime suspect.

Gastrointestinal Perforation: Although the patient has normal lipase, which might lean away from pancreatitis, a perforated viscus (such as a perforated ulcer) could still present with sudden onset pain and free fluid. The location of the pain (subxyphoid) could suggest a more upper GI source.
Hepatic or Splenic Injury: Trauma to these organs could result in free fluid in the abdomen. Even without a reported history of trauma, it's essential to consider this possibility, especially if the patient might not have disclosed or remembered a traumatic event.
Intra-abdominal Infection: Though the CBC is normal, early in the course of an intra-abdominal infection (such as a liver abscess or early peritonitis), the blood work might not yet reflect the severity of the condition.

Ruptured Aortic Aneurysm: Although less common in young individuals, a ruptured abdominal aortic aneurysm can present with sudden onset abdominal pain and is immediately life-threatening. The absence of typical risk factors (like older age, hypertension, or smoking) does not entirely rule out this diagnosis.
Ectopic Pregnancy: In a young man, this would be impossible, but considering the broad differential for abdominal pain and free fluid, it's a reminder to always consider the patient's sex and the specific presentation.
Ovarian Torsion: Again, not applicable to a male patient, but important to remember in the differential for acute abdominal pain in females.

Budd-Chiari Syndrome: This condition, caused by hepatic vein thrombosis, can lead to abdominal pain and ascites. It's rare and might not be the first consideration but should be kept in mind, especially if other diagnoses are ruled out.
Mesenteric Venous Thrombosis: Another rare condition that could present with acute abdominal pain and might lead to free fluid in the abdomen due to bowel ischemia or infarction.