What Is The Cause And Treatment For Severe Diarrhea?

I have had all kinds of tests. I have severe watery diarrhea with no answers. Illium was thickening in 1 scan 6 months ago but now it is not. Blood work comes back magnesium low , vitamin d low.. also stool fat is abnormal.. put me on ibs medicine and it does not work. Ige levels are high also..no one has an answer.