I fell on my knees three months ago, I rested for 24 hours after, but I have had pain in my knees since, resulting in 2 weeks ago that my knees were so painful that I could not stand or walk without any support. I have rested again for 2 more week, my left knee is now OK again with just the odd twinge of pain, but my right knee is still painful when getting up from the sitting position, and when first walking, after a while it eases off, but if I stand too long it starts to be very painful again. A lot of walking also makes it worse. I have had an xray of my knee and it does not show anything untoward. What can the problem be?