Commercial kegs come pre-carbed. So unless you vent and re-carb, pretty much no CO2 is going to enter the beer, you're just using it to push the beer out. Regardless, if it was really a contamination issue it definitely wouldn't go away after you poured one beer. I'd agree that it's in your head, or more likely it's something else causing the flavor. What kind of lines/faucets/etc? Were they cleaned? Sometimes if the beer sits in the line too long it can pick up a vinyl taste from cheaper beer line.