Welcome to HCM Forum ,
Considering your abdominal problem,concern that having major illness like cancer though no medical finding or doctor supported it,and having major concern about normal bodily functions with disturbed your routine,work efficiency definitely indicates that you having "Mental illness " ,most probably hypochondriasis
.This can episodic with change of symptoms over periods.
Due to certain neurochemical changes this kind of illness occurs,usually diagnosed by doing thorough investigation first to rule out any other illness (which done by you and your doctor assured you that nothing major physical illness you have with certain test result done by you are normal.)
This illness can be treatable.So visit to nearest psychiatrist
in person who after thorough evaluation and going through your investigation treatment will guide you further.
Hope this helps.