2002 Cadillac Deville Power steering?

I made a very sharp U turn after that when I attempted to pull into a parking space the steering was very hard, like when the engine ideled down, but when I kept turnong the wheel and applied more gas the steering returned to normal, once the car is moving even at 4-5 mph the steering si okay, and also no problem on the open road. When I did make the U turn there was a noticible groan from the front end. The belt looks fine and the fluid level is up.