What causes pain at the back of the knee?

Dr Sarah Jarvis MBE

A Baker's cyst is a swelling that can develop behind the knee. It is filled with synovial fluid which is the lubricating fluid that is usually found inside the knee joint. It most commonly occurs if there is an underlying problem with your knee such as osteoarthritis. Symptoms can include pain, swelling and tightness behind the knee. Rarely, a Baker's cyst can split open (rupture) and cause similar symptoms to a deep vein thrombosis (DVT). A Baker's cyst often gets better and disappears by itself over time. However, there are various treatments that may help if you do have symptoms associated with it.

https://patient.info/health/knee-pain-patellofemoral-pain/bakers-cyst