That's a good question and of course it made me smile :-)
A backache is NEVER just a backache, in the same way, an accident is never just an accident ...
Fear, kidneys and lower backache are all related. Lower back often relates to worry about finances or the fear of moving on because of lack based thinking.
The kidneys hold water when we are fearful. They constrict and retain. Retention is due to the fear of releasing limiting thoughts and old fears and then moving on to something new.