Food changes can cause GI upsets, though by 1 month, I'd expect those to be calming down.
Simple experiment is to add a probiotic such as Forti Flora or Intelliflora, or 1-2 teaspoons of plain yougurt with active cultures (if not dairy sensitive) to see if that helps.
If that doesn't, you'll need a vet visit.
Inflammation of the pancreas (pancreatitis) can cause damage to digestive enzyme formation which then results in soft, foul-smelling feces, usually a somewhat light pasty color. This is called exocrine pancreatic insufficiency and is treated with digestive enzymes. While there are some products over the counter, you really should have a proper diagnosis before starting these.
Bacterial overgrowth syndromes and parasites may result in stinky stool/diarrhea. Fresh fecal specimens may help in diagnosing this, so find out what your vet wants you to do.