What Causes Severe Diarrhea After Taking Amitriptyline?

I m 65 years old, I take multiple medications for back pain(methadone), high blood pressure, high cholesterol, & amitripylene for nerve problems in my back. Since being on this medicine I have had a fairly stable cycle of diarrhea about every 6-8 weeks(doctors all felt it was due to the medication). About 6 weeks ago my pain doctor put me on gabapentin for nerve problems in my back, within a week a began to have regular diarrhea instead of a normal stool. After 2 weeks of this I stopped taking the gabapentin and expected it to stop. It did not. I saw my pain doctor last week and she changed the gabapentin to amatripylene, the diarrhea has not stopped and I am not having some fairly severe gas pains and excessive flatulence. I have an appt. with a GI specialist in April but I m trying to get a handle on this because it is being disruptive as far as my full time job is concerned. Any thoughts I possible causes?