Aside from indicating sickness, grass munching may indicate that your dog is not getting adequate fiber from her diet. However, if you are feeding her premium dog food, you can rest assured that nutrient deficiency is not the problem. Your dog probably just needs to throw up, and grass is a sure way to trigger the response.
It is not as unusual as you may think, as this is a natural way for these animals to induce vomiting. If you notice this behavior alot, you may want to check his or her diet out for a better balance of meats and grains, as grains are a filler that dogs in the wild would only eat from the stomach of his or her prey. This may be a signal that there is toomuch grain/meal in your dog's diet.