From the symptoms you describe, I would think that hamstring cramps and impingments (muscles, vasculature, and/or nerves) are likely to be the cause. If you're listing the picking up of poo, you are likely aware of tension and feel that this might be a cause. Always follow up on your own suspicions. Of you have a lot of tension, dealing with that first (massage, stretching, relaxation) might be a good idea.