My mother must have cancer, but no doctor has been able to detect it. How do I care for her?

My mother is severly underweight and has been for nearly two years. She eats well and has no loss of appetite, yet she gains no weight. She is incredibly weak and dizzy ALL the time and there is not a month that goes by that she does not seem to go through some episode of some kind like a week long battle with diarrhea or pain in an area of her body or she gets a cold. It is always something. Granted, she has her setbacks and that could explain her weight staying so low, but she has seen every kind of doctor repeatedly. She has been tested numerous times for colon cancer. She has had Xrays and MRIs and dark masses have been detected, but they do a biopsy and say its only scar tissue. Could she be slowly dying of cancer, but it just cannot be detected? She must be fundamentally strong because she seems to still keep going no matter what. Anyone been through this?