I would suggest you look for a nutritional route instead of the drug nortriptyline due to the side effects. Usually it is used as an anti-depressant, but often anti-depressants are used for GI and other issues. I have taken it for the same reason many years ago and problems with urinary retention, (couldn't pee) because it interacted with some other things I was on.
I suffered with IBS terribly for 30 years and then I took Metamucil / Citrucil type vegetable laxatives, which helped tremendously, but in 2005, I found a better answer through patented nutrition which totally changed my life even more.
In a short time, I was medication free from many health issues and IBS is totally a non issue any more. I know MANY people who have had similar results with IBS, crohn's, ulcerative colitis, celiac, etc.
At the time I got on the nutrition, I was taking 19 different medications, from heavy narcotics to numerous over counter preparations. Today, I am still medication free, no longer disabled and feel better and am healthier than I was at 50, now am 65.
Most Drs. only know the pharmaceutical route for answers, and people are sicker than ever and going back for more meds. Unfortunately the medical community is so narrow minded and very slow to catch on that proper nutrition and diet is KEY!