Pain and burning sensation on knee

Hello,

The discomfort started months ago, but it has gotten worse. I have problems when lying down or sitting because I feel a sharp pain and a burning sensation inside my knee, just below the kneecap, slightly to one side. I dont find any good posture and standing up is the only solution I found.

The X-rays show nothing, but I have a feeling that this may be something else, as the discomfort is very severe. Could anyone tell me what it might be? I work sitting down and I don't know if that could be a cause.