This patient sounds as if he is suffering from hypovolemic shock. This is a common complication if abdominal surgery. Secondary complication could be internal bleeding. The diagnosis has to be based on patient examination and factual evidence such as studies. The assessment should include a head to toe with focus on the abdominal cavity. Lab should include a CBC and CMP this will rule out dehydration vs internal bleed. Next a CAT scan of the abdomen for bleeding, obstruction. No contrast should be used secondary to fear of perforation. The Doctor of course should be notified immediatly to obtain this orders and should have been contact at the first signs of distress with technically occured within the first hour of the patients care.