My dogs have done that alot exspecially my Newf they are different being a big dog now when i got her she was 6 months old and the breeder told me that they are not supposed to go down stairs until she is atleast 10 months point to my story we had to get arial rugs for our home because it is all hardwood and is not good on there hips and they dont have as much traction as the would walking around on carpet,She started limping around one day so we took her to the vets and his words were she most likely gave herself a charlie horse (easiest way he could explain it) i wouldnt worry too to much but then again we always do when it comes to our babies