Hi, Harish ji,
I will ask one question weather your wife got any injury
or trauma on the back ?
If there is no history she might have got stiffness
of back muscles due to sudden change of side or lifting up of baby or sudden stand up while sleeping.
She should take tab Dan MR twice a day with milk or with full stomach.
Do hot fomentation,
If there is no improvement then consult orthopedic surgeon on Monday,
Ok and bye.