Suggest Treatment For Sharp Knee Pain

about 12 months ago I turned over in bed and me knee seemed to wrench had pain/ache for a few weeks then it went away this happened a few times but this time (2 months ago) it hasnt gone away. The pain is spasmodic and if feels as if something is stuck in my knee, it gets worse and stiff after walking but no problem with walking. Sometimes in the night it hurts and I had a sharp pain at the back a few nights ago. I have been to my GP had an X-Ray which was clear and am to have an MRI scan on Sunday - any idea what this could be? my GP is not sure I am 63 weigh about 8st. 3 - medical history general good