Perhaps its time for the rod to come out. It could be anything as simple as the rod hitching against the tissues within OR mild infection OR loosening of the rod in the bone. If u had a recent fall, it could also be a fracture
. So, please get an X Ray
done and visit an Orthopaedician.
It would be wise if you visit the surgeon who operated on you 15 years back as he's more aware of what's going on inside than any one else.