Dirty Water Heater Exhausr

We packed up to come home today, and went around the trailer to find a black stresk of soot above the water heater vent and up the side of the trailer. We have never had this before. Behind the outside access door was some sooty flakes, and the diverter is black.

I have a stainless screen over the vent, so the likelyhood of something getting in is about nil. What would cause this... especially after alwsys burning clean?

Thanks.