What might cause pain at the back of the knee when bending?

When you bend your knee, several things inside the knee can cause pain behind it. One common cause is a Baker's cyst. This is when extra fluid builds up in the knee, often because of inflammation, and creates a small lump at the back. Inflammation (or synovitis) might happen from arthritis or from bending the knee repeatedly, which makes the tissues in the knee get swollen and sore. Also, the muscles and tendons around the knee, like the hamstring muscles or the popliteus tendon, can get stretched or strained. This can happen with sudden movements, sports, or everyday activities that put pressure on your knee. These tissues help support the knee, and if they are overworked or injured, you might feel pain when bending. It is a good idea to pay attention to how often it happens and how long it lasts. If the pain does not improve or makes it hard to move, you can use a free symptom checker to help identify possible causes before talking to a doctor about the best way to manage it.