Welcome to HCM,
You have not mentioned your age.If you are elderly then there is possibility of having osteo artheritic changes in your lumbo sacral region.
If you are young then it is possible to have heavy sprain
on your lumbo sacral spines.
Go for X ray
of lunbo sacral region and find out the cause.
Do back extension exercise,
Use hard bed for sleeping.
Do not bend forward and avoid weight lifting.
Ok and bye.