This new problem may have to do with your mother's low back. A condition called lumbar spinal stenosis--or LSS--is a common condition for people over 60. LSS is essentially a compression of the nerves in the base of the spinal canal. This compression causes a host of symptoms, such as numbness, tingling, and muscle weakness. However, two telltale symptoms of LSS that cause difficulty walking are low back pain and problems with balance.
A recent study showed that the majority of people with LSS had pain walking uphill. This may be due to added pressure on sensitive structures in the spine, such as discs.
A physical exam will give more specific information about your mother's condition. She'll want to see a doctor to find out what's causing her pain and what she can do about it.