80 pds loss of weight in 5 months is a significant loss of weight. One vital information which you failed to mention in your question is your age. There are plenty of reasons for pain in abdomen, nausea
and loss of appetite. It may be a benign thing like plain gastritis
or it can be as bad as a malignancy. Endoscopy
though is a good investigation of choice but then it can show only the hollows where the tube can go. it cannot show the status of your liver
and other abdominal organs. A CT/MRI scan are surely better in that respect. USG abdomen is another good option. I am sorry, but I couldn't understand by what did you mean by scan. If it is USG, then it is very safe and quick procedure, so please don't get worried.
Hope I am able to solve your concerns.