You have a disc herniation at L5-S1 below your fusion of L3-L5. Since your back pain never improved after surgery, it is unlikely that this new herniation is causing your current back pain. However, if the L5-S1 disc was degenerative prior to your fusion surgery above and was not suspected of causing pain, there is a possibility that is was also a pain generator but was not recognized as such.
Did you have a surgical work-up prior to your surgery for back pain? Did you have discograms or epidurals. If so, what were the results?
If you have a herniated disc at L5-S1, this could be causing your leg pain (assuming that you had no leg pain prior to the fusion surgery and the disc herniation occurred subsequent to the fusion surgery).