A change in bowel habits call for the following:
1) A physician exam
2) Proctoscopy; can be done in office
3) Sigmoidoscopy and colonoscopy; looking in the intestines by camera and scope
4) Barium meal studies
Following diagnostic considerations are in the review; diverticulosis, polyps, other growths like cancer, irritable bowel syndrome etc.
With an history of abdominal surgery, subacute intestinal obstruction may be considered which is very common because of the post surgery adhesions. Another not so common condition is rectal wall prolapse.
If the above investigations are normal, usually intestinal motility regulators, laxatives, high fiber diet is suggested. If these measures are not helpful, further investigations like colonic transit time and defecography may be pursued.
You need to consult a gastroenterologist for an evaluation and further management.
Please feel free for your follow up questions.