Thank you very much for the useful information Amelia.
Yes this could be related to gall bladder problem. Any obstruction to bile flow (such as stone or strictures) or reduced function of gall bladder can result in fat malabsorption which can give you abdominal cramps and diarrhoea.
Other than this, chronic pacreatitis (chronic inflammation of pancreas) can also give you similar symptoms typically after a fatty meal.
Then there are some other rare conditions of impaired fat metabolism which can give you these problems.
Glad to hear that your upper gastrointestinal endoscopy was normal, but do you know that at age of 50 you are due to have a colonoscopy? Its for screening against colon cancer (and a standard recommendation), so if not already done discuss this with your doctor.
You need tests for gall bladder such as ultrasound/HIDA scan as well as for pancreas (amylase/lipase) along with stool test for estimation of fat content.
If all these tests turn out to be normal, then yes we can attribute these symptoms to IBS, but its better to have this evaluated first in detail.
I would have been worried too if I were at your place, but let me reassure you that this does not sound serious at all. It is a treatable condition and carries a good prognosis with appropriate management.
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