Dogs are not carnivores they are omnivores so sometimes they eat small amounts of grass to substitute for any plant matter that they are missing from there diets.
My dogs will graze like cows and nothing is wrong with them.
They do eat grass when they have an upset stomach but I wouldn't worry about it unless your dog becomes lathargic, goes of its food and/or is drinking much more water than normal.
Hope this helps.
JoeD