Back Pain & Inability to turn on bed.

My mother aged 55 is a home maker. She has been experiencing severe back stiffness and backache for the past 5-6 months. Her inability to turn over in bed from a particular lying position has drastically been affected. It gets extremely difficult for her to turn around in bed once she has lied down in any position. Also it takes her nearly 5-10 minutes to get up from bed. If she wakes up from bed within 30 mins of lying in bed the backache gets severe for the entire day. Could you please kindly direct me to the correct diagnosis and possible treatments that I should be looking at.  Which doctor shall I be referring to a neurologist/ orthopedic/ rheumatologist. Kindly guide me in this regard. P.S. : She is diagnosed with Gallstone of 8mm for the past 1 year (earlier it was 12mm and it has reduced in size when last diagnosed). She does not have any other co-morbidities.