I'm not very familiar with colon cancer, but there are several serious digestive system diseases that can cause some of the symptoms you mentioned. Getting a quick and accurate diagnosis for older folks seems to be more difficult, often because older folks sometimes suffer from more than one condition at a time. So unravelling the symptoms into their correct possible causes is something like straightening out spaghetti while it's still cooking.
Some other diseases that you might look up include: Crohns Disease, Ulcerative Colitis, and pancreatitis. If you have a list of the tests that have been run on your grandmother, this can help you figure out which specific diseases her doctors think might be possibilities.
Here are some links where you can find reliable medical information: