2000 Honda Civic Exhaust Fumes

I recently had the timing belt and water pump replaced for about $450. I noticed shortly thereafter that when I turned on the fan for heat that heavy exhaust fumes filled the interior. I took it back and was told a valve cover gasket needed to be replaced for another $115. I'm curious to know whether this was just a coincidence or if the work done to replace the timing belt and water pump might have been the cause?