After the operation during which colon cancer was removed, it is possible that a minor local inflammation due to damage of the tissue might have occurred. The re absorption of the secreted fluid might have not been complete. The residual fluid could have thickened into connective tissue that connects the intestines. Contraction of one part of the intestines pulls another part and causes pain.

Another possible reason for the pain that your sister is experiencing might be that the process of bowel movement has not been completely restored, thus causing more gasses to inflate the intestines and to cause pain. If this is the case, you might try some over the counter antispasmodic products. Either way, I suggest that you consult with the supervising oncology surgeon in order to report the symptoms and seek a diagnosis.