My legs have been feeling weak from the kneecap down to my ankles when i walk and when i sit down, it feels like there is pressing in my leg. causes?

Several: 'burning' knee painwithout any injury may be caused by tendinitis( patellar, quads, it band etc.) or from kneecap (patellofemoral) cartilage wear most commonly. See your orthopedic surgeon if your pain continues for examination. In your case patellofemoral instability can cause pain, 'popping', and a feeling of instability. Quad strengthening and bracing may be the initial treatment. Best of luck!