I agree with MrTomD. This is just unburnt fuel igniting in the exhaust. High performance cars that do not have catalytic converters can do this. I don't know if I mentioned this elsewhere, but friend of mine had a Porsche that would do this if you let off the throttle. It would shoot flames out the back on shifts. Pretty cool. I drove it one time in town and got on it and a lady pulled up beside me and said the car was on fire. LOL.
If this were happening on acceleration, or when it was starting up I think you'd have a problem, but I think you've got nothing to worry about. Let us know if things change and it happens elsewhere. Good luck!