With their been a weight-bearing element to your complaint I'd be tempted to say that something is going on on/between your articular surfaces. Can you actually 'touch' where the pain is or does it feel much deeper? Also, how would you describe the pain? Sharp, dull etc.

Chances are, your body is still recovering from the surgery. I had a similar experience. Just keep an eye on it, stick to rehab and don't be shy to contact your consultant if things don't change!