I Am 46 And Active In Sports And I Did

I am 46 and active in sports and I did something to my right knee. It hurts on the right side and there is a lot of cracking when I move it. I cant cross my leg anymore it hurts to bad. It is not hot and doesn't swell. I have already had my Orthopedic doctor look at it and do a X-ray. He said if keeps hurting and the pain is to much to handle than lets get an MRI, Doc the pain goes down my leg. What could this be? My strength is there still but the knee feels a little loose and cracks when I move it, any idea Doc?