Back Pain after surgery?

I had surgery 3 years ago on my L5 S1 -they replaced the disc and fused it. I was ok for maybe a year. After that I have gone to a pain clinic because they found I had a degenerative disc in my cervical spine. I also have pain still in my lumber spine- although the doctors say that my MRI's don't say anything. I just started getting pain on the right side of my lower back and down the back of my thigh. Has anyone had this problem after surgery? Does anyone have any suggestions as to what this could be?