Other than diet changes what could cause my stool to be less firm and far more smelly?

Lot of possibilities: Diet changes would be the most common cause, other possibilities would be lactose intolerance, irritable bowel syndrome, gluten sensitivity, problems with your pancreas, or an intestinal infection. Give it some time and notice any pattern with your diet or any exposures to others with diarrhea, if it doesn't get better in a week or so, or if you have fever, pain, or dizziness, see a doctor.