Baker's Cyst (Popliteal Cyst): This condition is characterized by a fluid-filled swelling that causes a bulge and a feeling of tightness behind the knee. It is often associated with knee flexion, especially at end range, due to the increased pressure on the cyst.

Gastrocnemius or Hamstring Strain: Strains in the muscles at the back of the leg can cause pain, especially when the knee is flexed, as these muscles are stretched.
Popliteus Tendinitis: Inflammation of the popliteus tendon, which is located at the back of the knee, can cause pain when the knee is flexed, particularly at the end range of motion.
Meniscal Tears: Tears in the cartilage of the knee joint can cause pain, especially with deep knee flexion, as the torn meniscus can become pinched.

Deep Vein Thrombosis (DVT): Although less common, DVT can cause pain behind the knee, especially if the clot is located in the popliteal vein. This condition is a medical emergency due to the risk of pulmonary embolism.
Popliteal Artery Aneurysm: An aneurysm in the popliteal artery can cause pain behind the knee due to compression or thrombosis. It's a rare but potentially limb-threatening condition.
Tumor (e.g., Soft Tissue Sarcoma): Although rare, tumors behind the knee can cause pain, especially with movement. Early diagnosis is crucial for effective treatment.

Cystic Adventitial Disease: A rare condition where a cyst forms within the wall of an artery, leading to pain and potentially affecting blood flow.
Intraosseous Ganglion Cyst of the Tibia or Femur: A benign cyst that forms within the bone, which can cause pain, especially with movement.
Neurogenic Tumors (e.g., Neurofibroma, Schwannoma): Tumors arising from nerve tissue can cause pain and other neurological symptoms, depending on their location and size.