Hi, thanks for writing to HCM.
By the description, it could most propably be due to a muscle pull or a osteoporotic compression fracture
of vertebra (unlikely).
Xrays will usually show up any bony injury
and is always mandatory in elderly people complaining of backpain.
Temporarily apply some muscle relaxant
gel, Hot fomentation and bedrest
I suggest you to consult an orthopedic surgeon for further workup.
Hope this helps