Patellofemoral Pain Syndrome (PFPS): This condition is characterized by pain in the front of the knee, usually around or behind the patella. It is often exacerbated by activities that put stress on the knee, such as sitting, squatting, or climbing stairs. The pain typically improves with movement, making PFPS a strong candidate given the symptoms described.

Iliotibial (IT) Band Syndrome: This condition occurs when the iliotibial band, a ligament that runs down the outside of the thigh from the hip to the knee, becomes inflamed. It can cause pain on the outside of the knee, especially when the knee is bent for an extended period, such as when sitting. Movement can help alleviate the tension and thus the pain.
Osteoarthritis (OA) of the Knee: While OA can cause pain at rest, it often worsens with prolonged inactivity and improves with gentle movement. However, the pattern can vary, and some individuals might experience relief with movement after initial stiffness.
Bursitis: Inflammation of the bursae around the knee can cause pain that may be worse with pressure or prolonged positions like sitting. Movement can help reduce the pressure on the affected bursa, potentially offering relief.

Deep Vein Thrombosis (DVT): Although less common, DVT can present with pain in the knee or leg, which might worsen with prolonged sitting due to increased venous pressure. It's crucial to consider DVT due to its potential for severe complications, such as pulmonary embolism.
Septic Arthritis: This is an infection within the joint space, which can cause severe pain, swelling, and stiffness. While it might seem less likely given the improvement with movement, early stages or certain types of infections could present atypically, making it a critical diagnosis not to miss.
Tumor: Bone or soft tissue tumors around the knee can cause pain that may have a variable relationship with activity and rest. Although rare, missing a diagnosis of a tumor could have significant implications for treatment and prognosis.

Osgood-Schlatter Disease: This condition affects the tibial tubercle, causing knee pain in adolescents. It's more common in young athletes and might not perfectly fit the described pattern but could be considered in younger patients.
Sinding-Larsen-Johansson Syndrome: Similar to Osgood-Schlatter disease, this condition affects the patellar tendon at its insertion on the tibia, causing pain below the kneecap. It's more typical in young athletes and might present with pain that varies with activity.