Why do I have loose light colored stool that smells very bad?

Stool light-colored. Bile & your gut's bacterial flora account in large part for color & volume of stool. Absence of bile results in pale, clay-color stool. Lots of other things also affect stool appearance: ingested foods, medications (peptobismol, antacids), swallowed blood or bleeding from gut, infection/antibiotics, inflammation, maldigestion, & the amount of mucus mixed with stool, as well as transit time of food.
? malabsorption. In addition to the issues mentioned by dr. Cattano, malabsorption can produce the symptoms you described. If you have lost weight, it is imperative that you consult your doctor.