What Causes Chronic Persistent Pain In The Back, Buttocks And Knee?

I am a 47 year old male. I have had pain in my back, right buttock, and right leg/knee for several months. Sometimes the pain is severe. I can hardly stand on my leg at times.. it is like my leg gives away. I have have an MRI of my back and and MRI of my knee, and there is nothing that the orthopedic doctor can see that should be causing this pain and instability. X-rays look OK, too, besides a little arthritis in my knee. The MRI of my back shows that there is a small bulging disk on the left side, but the doctor said that it should not affect the right side, where the pain is. The pain moves around from my leg, to my knee, to my buttock. A few days ago, the muscle in the top of my leg was extremely sore. This was after I picked up a generator and severe back pain. I have taken two rounds of steroids a a few months ago. They helped for the first couple of days, and then then pain came back. I was given a steroid injection in my knee with numbing medicine in it, and that helped until the numbing medicine wore off. I am now on Mobic, but there has not been a big change in my pain. Any ideas on what could be going on? Thanks for your insight!!