A 46 year old woman presents with sudden episode of abdominal pain which started about the most appropriate investigation for the cause of the patient’s pain?

A 46 year old woman presents with sudden episode of abdominal pain which started about 2 hours ago. The pain is located in the epigastrium and radiates to her back. She has vomited twice since the onset of attack. The pain is made worse by lying flat on her back and she is more comfortable sitting up and bending forwards. She was informed of the presence of gallstones in her gall bladder four weeks earlier when she reported pain in the right hypochondrium. The oral temperature is 39C, BP 120/80mmHg and the radial pulse 118/min. There is no jaundice but there is marked tenderness in the epigastrium both on deep and superficial palpations. Which is the most appropriate investigation for the cause of the patient’s pain?