There area number of health issues that could cause this.
One might be a problem with the spine particularly in the neck or lower back area where pinched nerves cause a loss of sensation and control due to disc issues.
Another might be due to a heart problem where the heart can't supply the muscles the blood flow they need to keep working well.
There are muscle diseases that can cause this type of exercise intolerance.
Difficulty in breathing can also lead to exercise intolerance issues.
If its the rear legs then hip dypslasia or a knee problem might be the issue.
I would plan on a vet check up for her and in addition to the physical your vet may want to test blood work or do some x-rays or both.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics