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Oancreatic cancer is very unlikely.
By 18 months pancreatic cancer would have spread all over and would have been apparent in all these tests.
These symptoms can also be due to the gallbladder problem.Excess bile acid will cause intestinal irritation.When the gallbladder is not functioning well, the bile flow to the intestine may increase causing these symptoms. This will result in bloating, belching and also diarrhea especially after meals.These symptoms can be treated with a medication called Cholestyramine 4 gram twice daily will meals. Malabsorption syndrome is another strong possibility.Malabsorption syndrome includes the follow possibilities:1. Excess bacterial overgrowth in the small intestine known as SIBO syndrome.This is not usually diagnosed by the colonoscopy.Small intestinal aspirate can be sent for bacterial count.Carbohydrate breath test ( D xylose) is a simple test to diagnose this condition. But, the treatment is much easier.A 14 day course of Rifaximine improves the symptoms dramatically/2. Lactose intolerance--- regular milk, dairy products. 3.gluten senisitivity or celiac disease--- Wheat based products. 4. Chronic pancreatitis --- deficiency of the digestive enzymes produced by the pancreas. You should consider following low fermentable sugar diet ( FODMAP diet) to reduce your symptoms. Please go to this link for detailed information:http://stanfordhealthcare.org/content/dam/SHC/for-patients-component/programs-services/clinical-nutrition-services/docs/pdf-lowfodmapdiet.pdf
Please also include prebiotics like Inulin daily to keep your colon acidic which prevent gas producing bacteria. Also take a probiotic ( lactobacilli, bifidobacterium) daily. Lastly, there can be chronic infections like C difficile, Giardia, Amoeba, Aeromonas etc. These infections may be diagnosed by checking 3 stool samples taken on successive days. They will require proper antibiotics for the treatment.Please discuss these possibilities with a gastroenterologist.
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I believe your problem lies in dietary intolerance.
It is better to try out the above recommendations which help immensely in conditions like yours.