Usually none. A fusion is indicated usually to control pain in certain instances, to correct a deformity, fix an instability or a fracture or sometimes to treat an infection. A fusion that has healed should not be causing pain unless there is bone growth pressing on a nerve, the fusion did not heal or instrumentation if used with the fusion may be impinging on a nerve.
Timing. Pain after having a fusion depends on multiple items: such as how long it has been since your surgery, if the pain is at the donor site for the fusion, and if you had degeneration previously. I would imagine this is related to a spinal fusion and you are continuing to have pain after your fusion healed. If you had degeneration at other levels prior to surgery; this pain can be from those levels.