Leg and knee pain

I have been having back pain for about two years now after injuring it.  I have had X-ray's done and an MRI done, both have come back normal.  The back pain was in the middle of my back and I never had any leg and or knee pain because of it.  The first day that I had pain in my leg, it was the left, the pain was barable with medication.  I went bed that night and the pain shifted to my other leg by the time I awoke the next morning.  The pain is so intense that it is hard to walk or put any kind of pressure on the leg.  The pain is not in the back of my leg but rather in the groin area and shoots down to my knee.  What is going on here?