What Causes Severe Diarrhea

My 86 year old mother has had serious diarrhea for weeks. She is dehydrated and potassium is 3.14. ER visit, 2 bags IV S with Potassium. Cdiff negative. Dizziness, No fever, no flu like symptoms. She lives in an assisited living facility although in a private apartment. Takes one meal a day in dining room. Doctors cannot find a cause. All stool samples are negative.