Cat Converter/Exhaust Manifold?

After getting a code for a bad fuel injector, I replaced all fuel injectors. Several days later I noticed the truck smoking. It appeared to be smoking right at exhaust manifold/ Cat Converter on driver's side. After looking at it closer, it appears to have oil residue right where the Cat begins around the heat shield. It is pretty baffling how oil is right there. No evidence of oil leaking anywhere, no loss of oil. I'm baffled and worried if I replace the Cat converter will I still have a problem? Where/Why is there oil in this specific location?