Osgood-Schlatter Disease (OSD) recurrence: The patient has a history of OSD, which is a common cause of knee pain in adolescents. The fact that the pain returned after initial improvement with physical therapy suggests a possible recurrence, especially given the patient's age and the bilateral nature of the pain.

Patellofemoral Pain Syndrome (PFPS): This condition is characterized by pain around or behind the patella, often related to activities that put stress on the knee. Given the patient's age and the return of knee pain, PFPS is a plausible diagnosis.
Overuse or repetitive strain injuries: The patient's return to activities that may stress the knee could lead to overuse injuries, which are common in adolescents.
Growing pains or idiopathic knee pain: These are common in adolescents and could be a consideration, especially if other causes are ruled out.

Tumor or cancer (e.g., osteosarcoma, Ewing's sarcoma): Although rare, it is crucial to consider malignant causes of knee pain, especially if the pain is persistent, severe, or accompanied by other systemic symptoms like weight loss or fever.
Infection (septic arthritis or osteomyelitis): Infections can cause severe knee pain and must be considered, especially if there are signs of infection such as redness, swelling, warmth, or fever.
Rheumatologic conditions (e.g., juvenile idiopathic arthritis): These conditions can cause chronic knee pain and should be considered if there are other systemic symptoms or signs of inflammation.

Osteochondritis dissecans: A condition where a piece of cartilage and bone in the knee joint becomes damaged, leading to pain and swelling.
Meniscal or ligamentous injuries: Although more common in older adolescents or adults, these injuries can occur in younger patients, especially if they are highly active in sports.
Chronic recurrent multifocal osteomyelitis (CRMO): A rare condition characterized by recurring episodes of bone inflammation, which can cause pain and swelling in the affected areas, including the knees.