What causes burning pain in the back of the knee? what could the pressure be from? hurts to bend or straighten the knee. please i need an explanation

Multiple things: could be a multitude of things: 1 - DVT. A blood clot typically arises from the lower leg veins and can cause pain and swelling. The test to diagnose this is a duplex ultrasound of your leg. 2- Musculoskeltal- could be arthritis, a ligamental strain, or some other tendon/ligament/bony issue, or a Baker's cyst 3- Vascular issue. Sound like you need in person evaluation. Hope this helps.