Pain isn’t always identifiable. The causes can be hidden or the pain can be psychosomatic, or in the mind. However, that does not mean that the pain doesn’t exist, just that it’s not physical.
If it is possible, try to get your mother referred to a pain clinic where she can be assessed again about what may be causing the pain. If they can’t find any reason for the pain, they still may be able to provide options for her to get some relief. There could be medication or even counseling to help her manage the pain or to find ways to work around it.
Keep advocating for your mother; there are ways to help but you may have to be persistent and do more looking around.