Dull knee pain

Some days it feels dull, and other days it doesn't.

But a few days ago, it was aching, so I went to the hospital.

They did an X-ray and ultrasound exam.

But they say there is no abnormality.

I have no symptoms, so why do I feel sick?

Is it possible that this is temporary? I'm in my 40s now, and I'm worried that my joints might already be deteriorating.
Would it be better to try switching to an oriental medicine clinic?