If you have a lump behind your knee that's filled with fluid and causes a feeling of tightness, it might be a Baker's cyst. Doctors also call it a popliteal cyst. The pain can get worse when you fully flex or extend your knee or when you're active.
It is usually the result of a problem with your knee joint, such as arthritis or a cartilage tear. Both conditions can cause your knee to produce too much fluid, which can lead to a Baker's cyst.
For more info you should go to a specialist.
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