It is very possible that bile salts are causing your diarrhea. Removal of the gall bladder sometimes causes the liver to produce more bile salts every time you eat. Some people produce so much the small intestine cannot reabsorb them. This causes diarrhea. I suggest you read the information at this link: http://www.gihealth.com/html/education/bilesaltDiarrhea.html
It's possible that your GI is correct that the gall bladder removal is the culprit. I am surprised that if he thinks that's the case that he didn't prescribe one of the medications that contains cholestyramine resin. The different brands are listed in the article I linked.
Hope this helps.
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CD, Ankylosing Spondylitis, lupus, small fiber peripheral neuropathy, avascular necrosis, peripheral artery disease, degenerative disc disease, and a host of other medical problems.