It's hard to say, but this isn't a typical symptom of congestive heart failure. Usually with heart failure, you'd have exercise limitations, shortness of breath, and swelling in the legs.
Your mother's doctor should definitely evaluate her. In this case, a physical exam would probably be the most useful thing, to look for evidence of worsening heart failure and neurologic abnormalities in her legs. If the neurologic abnormalities are worse than at her last exam, an MRI of the spine might be indicated to confirm that stenosis is the cause.
Since she's not losing consciousness or feeling dizzy or light-headed, it makes things like arrhythmias common with CHF less likely. I'd guess that her spinal stenosis is the likely cause, but I'd need a lot more information to say for sure.