I am not a vet, but I run a boarding cattery and we have come across this occasionally in cats.
One cat was so bad that we actually took it to the vets, and of course when we got there the cat walked perfectly normally.
Our own cat had an operation on its leg a couple of years ago and usually runs around the garden with no problem, but sometimes we can detect a slight limp when he walks.
It could be that your dog's leg goes to sleep, as in humans.
Or at her age it could be a touch of arthritis, which becomes worse after exercise, again just like us.
I understand that you don't want to stress her out,but a short vet visit will put your mind at rest and, if it is just something like arthritis you could get some pills or other medication to ease her problem.
You can also purchase supplements, like glucosamine, to give your dog, which may ease the joints.
Pop her down to the vets, the trauma won't last long, she'll soon forget it and forgive you and you could ease her problem for her.