If it has variable effort steering it might do the same thing again in the middle of a high speed maneuver in traffic. Be ready! Have somebody with a GM TECH2 or similar code reader capable of reading out all the trouble codes and not just engine codes, check for steering codes stored in history. This is a very dangerous situation.
I am sorry to hear that you are having concerns with your Cadillac STS. For clarification, did you recently purchase this vehicle, or did you have it serviced? Also, have you considered visiting your local dealership to get a diagnosis on this? If so, could you please send me a private message with your contact information, VIN, and preferred dealership so we can help you further with these concerns.