Usually you have to seek a referral from a general practitioner, so I'd just take her to an appointment with her regular doctor. It could be arthritis, age, etc. It's hard to assess through the internet
Arthritis can cause these symptoms and can improve with movement. Certain types of back strain or problems with disks can also cause these symptoms. Even spasms can cause similar symptoms. She should see an Orthopedic doctor who specializes in bones and joints. He/She will likely look into an MRI or other type if imaging to evaluate your mom's back to check for slipped disks or other types of injury or strain and make recommendations from there.
Hope this helps!