What Causes Progressing Pain In Left Leg?

I have pain in my left leg. It has been getting progressively worse over the past 7 1/2 years. I am a 57 year old female, 125 pounds who was in good shape. I cannot stand in 1 position for very long. It started behind my knee & now it is up to my hip. I have tried every possible therapy & have had MRIs of my thoracic spine to my lumbar spine. Nothing showed up.