Eight and a half years of your chicken-rice- Science Diet diet seems to have
brought her along in good fashion.  Is eating grass a new "treat" ? If it is
the only new addition to her diet, that may be the cause?   Look for
anything new she may be "adding" to her diet on her own.  Have any chemicals
(fertilizer, weed killer etc.) been used on the area she roams and "grazes"?
Have you changed the ratio of chicken, rice, S.D. ?.
  As dogs age things sometimes change in their body and it's function.  If
the vet thoroughly checked her out and found nothing, that's a good thing.
Keep an eye on her stool for any changes   such as blood or "real slimy".
Also parasites might cause this?
When my Dales had this problem, a change from dry food to  a temporary diet
of rice and cooked hamburger usually got them back on track
For what it is worth I hope this might help.
Good Luck
Mike