Explanation?

Hi! This is quite random but, I have been diagnosed IBS and take Smooth Move tea as per instructed by a doctor. In the mornings, when my stool is normal and "solid," my weight is usually normal or even down. If my stool is liquidy and also "diharerea-like" my weight will be up. Even when my stool is solid and I may not have fully gone, it is still lower than when I fully empty my colon from loose stool. Anyone have any insight as to why this may be? You would think it would be the other way around.
Thanks!