Could be a number of possibilities....
a) Some fumes from one of the engines (after start) were "inhaled" by the air conditioning system
b) The APU when shut down sometimes leaves a slight smell as you described, into the air con system
or
c) you could have been right behind another aircraft with its engines running and it's fumes were "inhaled" by your aircraft's air cion system.
All perfectly normal, and quite a regular occurrence.
Nothing to be worried about !