You have a reported “extra vertebra” in your lower back. This could be a sacralization of L5 or a lumbarization of S1. When we are in utero, the sacrum is actually 5 separate pieces of bone and fuses together when we are a fetus. Sometimes, the message gets confusing and the L5 vertebra might try to fuse to the sacrum or vice versa.
Typically, this “extra” level is stable and not capable of becoming painful. There are rare circumstances that a joint called a transverse-alar articulation between these segments can cause pain, but this is unusual.
You need an MRI of your back and a consult from a good spine specialist.
Dr. Corenman