You didn’t do anything wrong and she may have not even been shaking her head at you for anything. Reading too much into these things is a surefire way to drive yourself insane and increase anxiety. You were off the clock and if policy states you are not supposed to work off the clock then you did the right thing. I’m not sure if helping your co worker in that instance would have been considered working. Probably not. However she didn’t ask for help and you were shopping. Nothing to worry about here. If you are that concerned about the co worker shaking her head then you can simply ask her if it was directed at you.