My Mother Has Been Ill With Diarhea Off And On,

My mother has been ill with diarhea off and on, mostly on, for more than 10 months. She has seen several GI specialists and a primary care physician and they cannot find anything wrong. meanwhile she has lost 25 pounds and weighs 104 lbs and is losing hope. Could this be lupus? One GI specialist said IBS, and the other contradicted him. Does Lupus come on at age 81 and could it be missed by experts testing her?