Dozens of stools a day and the MD says don’t try to stop it?!

My doctors seem to have the notion that I get these attacks because of underlying constipation, so I should not take pepto or immodium or the cycle will be continue. But a full day of near-constant diarrhea leaves me constipated later anyway. Today I am having one of the worst attacks ever and this is AFTER a week of going every day or even several times a day. Is there no other theory about what is causing this?! (Have normal colonoscopy, stool tests, etc?)