Most probably she would be suffering from radiation induced proctitis which at times trigger passage of stools often it is better you get consulted with the nearest gastroenterologist and you need to undergo a sigmoidoscopy or colonoscopy to see the nature of the anastomotic site and you need to do a CT abdomen scan to look for tumor spread and efficacy of the treatment done you may be additionally be subjected to certain blood test which is called as tumor markers.

Please take a physical consultation from a local gastroenterologist