After pooping? So, you are sitting for a bit? Does it take you awhile to go? :>) If this has come on suddenly, it is usually due to injury. Has anything happened recently? You could have a cartilage tear in the back of your knee (meniscus tear), you could have a deep vein thrombosis (blood clot), you could have arthritis of your knee, things like your calf or hamstring could be having issues (too tight) and it is pulling on it. Arthritis is the most common reason to have pain after getting up after sitting. Bottom line, many things can cause it and you need a doctor to evaluate. DVT is something you'd want to rule out right away. good luck and let us know!!