When the back of the knee becomes swollen with fluid in this cyst it can be irritating and painful. The knee feels full and you can get pain from pressure on the nerves and tendons. The cyst is usually linked to the knee joint and the swelling can be part of a problem within the joint itself from ligament, cartilage or meniscus issues. A recurrent cyst needs to be fully assessed as part of a complete knee examination to determine the underlying cause. Unless you get a proper diagnosis then they often recur.