What is behind knee pain? Behind knee pain is discomfort or soreness behind the knee joint. It may occur with or without movement and can be severe enough to limit movement. People may describe behind knee pain as discomfort, inflammation, increased warmth or burning, soreness, stiffness, or pain. Behind knee pain is a common sign of a Baker’s cyst, which is a buildup of synovial fluid behind the knee. Synovial fluid lubricates the joints and reduces friction between the joint cartilages. The symptoms associated with a Baker’s cyst tend to come and go, and the condition will not cause any long-term damage to the knee. However, behind knee pain may be a symptom of deep vein thrombosis (blood clot in the leg), which is a serious and life-threatening condition. The blood clot can break loose and cause a pulmonary embolism in the lung, a heart attack, or even stroke. Deep vein thrombosis has similar symptoms to a Baker’s cyst. If you experience pain behind the knee and calf with painful swelling and bruises, seek immediate emergency medical care. If your symptoms persist or cause you concern, contact a medical professional.