Yes, I would check with her doctor. It could be a number of things.
My cousin, who has dementia, got very stiff with dementia. She also had arthritis, so it's difficult to know what is the main cause, but she is very stiff in her legs when trying to stand. She can't walk. But, she can move them pretty well when sitting in a wheelchair. You might take note of your mom's ability to move in various positions and see if anything stands out, so you can share with the doctor.
My cousin, who has dementia, got very stiff with dementia. She also had arthritis, so it's difficult to know what is the main cause, but she is very stiff in her legs when trying to stand. She can't walk. But, she can move them pretty well when sitting in a wheelchair. You might take note of your mom's ability to move in various positions and see if anything stands out, so you can share with the doctor.
She may be frightened, it could be she gets dizzy..