The fermentation process creates a lot of gasses that remain in suspension to some degree. Changes in pressure, bumping the vessel, temperature can all affect their release from suspension and that is what you are seeing, off gassing. perhaps in drawing the sample you bumped the vessel or removal of the air lock changed the pressure within. What ever it is I would not worry about it and it is not carbonation of the beer.
For CO2 production you either need to add sugar for the yeast to consume or introduce it by kegging it.