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Since all the tests for diarrhea has been negative, diarrhea predominant Irritable Bowel Syndrome
is one of the possible causes.
You have not mentioned how long you have been having these symptoms and whether you have abdominal pain
or discomfort and whether it gets relieved after defecation.
All these details are to be known before considering a diagnosis of IBS-D.
You have also not mentioned whether small bowel diseases/malabsorption
syndromes like celiac disease
have been ruled out.
IBS-D is a disease of exclusion and all the possible diseases should be ruled out before calling it as IBS-D