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Left sided abdominal pain can be due to possibilities as:
1)Peptic ulcer disease or Hiatal hernia. Diagnosis can be confirmed by upper GI endoscopy.
2 The second possibility is of pancreatitis as it may cause upper left abdominal pain which is worse when bending over or sitting. Pancreatic enzymes like serum amylase and lipase should be measured to rule out any pancreatic cause.
3)The last possibility is of any kidney stones which may present as sharp pain radiating to the back.
As CT scan result is relatively normal it might be suggestive of Inflammatory bowel disease as Chron's disease or Ulcerative colitis. Another cause can be lactose intolerance and you would have to check if certain foods cause diarrhoea. It is difficult to give a precise diagnosis without clinical examination. Kindly visit a Gastroenterologist for thorough evaluation.
Take Care!