Many studies have shown that spinal fusion puts increased stress on the adjacent level (above or below). It's not uncommon for problems to occur later at the next level.
About two to three per cent of the patients who have cervical (neck) fusion develop new symptoms of neck and/or arm pain each year. Disc degeneration is the most common cause of the symptoms. Spinal joint arthritis and joint instability can also lead to similar problems.
If there were changes at the C56 level at the time of your first operation, the X-rays would have shown them. Usually it's the case that X-rays appear normal before surgery but changes occur after the fusion.