Similar problem to you, that occurs at various times of the year, normally after a great deal of physical inactivity in the office when I am tied to my chair for prolongued periods. Get a doctor to diagnose and hopefully treat, but get it done. Like you, bike riding is not affected, and in fact stretched out on the bike is one of the few comfortable positions for me. Mine is a compressed disc by the way, and does not improve by sitting down or severe bending to do the gardening. Get back on the bike and get some relief, if only for a few hours.