Finding the source of pain behind the knee can be tricky due to the knee's complex anatomy that includes bone, tendons, fluid, ligaments, arteries, and veins. Behind the knee, there is a large vein that, when blocked, can cause swelling that is hot to the touch and sometimes painful.
Blood clots tend to form here because the knee sits low in the body and is crimped when the leg is bent, sometimes for hours on end. This prevents the one-way valves in the vein from flowing freely back up to the heart, causing blood to pool and become prone to clots. If these clots should break away and travel in the bloodstream, they can lodge in blood vessels elsewhere - in the brain (causing stroke), lungs (causing lung embolism) and heart (causing heart attack).