Answer:
Since it seemed related in time to the cancer surgery, it might be a result of that. If you have spasms, slowed intestinal transport, or constipation, there may be partial bowel obstruction due to scar tissue.
More likely, you have some type of intestinal malabsorption or food allergy such as lactose (milk sugar) intolerance, wheat or gluten intolerance (celiac), etc., which can come on fairly suddenly from early middle-age on.
I would suggest seeing a GI specialist (gastroenterologist)as a first step in evaluating your symptoms.