A blood clot or deep vein thrombosis (DVT) can cause pain behind the knee. “And that is a very concerning cause because the clot can go to the lungs and be very dangerous,” Dr. Packer says.

They stop the protein from making DVT blood clots in your body. Some people get it to prevent DVT if they've had a hip fracture, hip replacement, knee replacement, or abdominal surgery.

On the other hand, venous clots can move through the bloodstream and lodge in small blood vessels in the lungs. “That is an embolus. Ninety percent come from below the knee and move upwards ...