Agreed. Your snake will poop when she needs to. You may be right about an upcoming shed, they will often refuse food and hold on to their poop if a shed is coming. Keep your eyes open for a punk belly, dulled skin and cloudy eyes.
Also, what type and size prey are you feeding and how much does the snake weigh? If the meals are too small the entire prey item may be used to sustain growth, so there may be nothing left to poop out.