Baker's Cyst: This is a fluid-filled swelling that causes a bulge and a feeling of tightness behind the knee. It's often associated with knee arthritis or a knee injury, and walking can exacerbate the pain.

Hamstring Strain: The hamstrings are muscles at the back of the thigh, and straining them can cause pain in the back of the knee, especially when walking or bending.
Calf Strain: While more commonly associated with pain in the lower leg, a severe calf strain can refer pain to the back of the knee.
Popliteus Tendinitis: Inflammation of the tendon of the popliteus muscle, which is located at the back of the knee, can cause pain in this area, especially with activities like walking.

Deep Vein Thrombosis (DVT): Although less common, DVT can cause pain in the back of the knee, especially if the clot is located in the popliteal vein. This condition is potentially life-threatening if not treated promptly.
Popliteal Artery Aneurysm: An aneurysm in the popliteal artery, which runs behind the knee, can cause pain, swelling, and other symptoms in the back of the knee. Rupture of the aneurysm is a medical emergency.

Popliteal Cystic Adventitial Disease: A rare condition where a cyst forms around the popliteal artery, leading to compression and potentially causing claudication (pain in the leg when walking).
Gastrocnemius or Popliteus Muscle Tear: Tears in these muscles can cause pain in the back of the knee but are less common than strains.
Neurogenic Causes: Such as a lumbar disc herniation or peripheral neuropathy affecting the nerves that supply the back of the knee, can also cause pain in this area but would typically be accompanied by other neurological symptoms.