Generally dog seat grass when they have a upset stomach, but this is not good for her because not only will it make her vomit but can cause blockage. If she becomes lethargic or you notice blood in the stool or vomit, she needs to see her vet promptly. As this is a sign of serious illness including a parasite infection or a blockage among other things. If your dog is unvaccinated she will need vet attention as this is a common symptom of some very serious illness.
I would start her on a bland diet of boiled chicken and rice to help settle her stomach. I recommend giving her unflavored pedilayte If she begins vomiting or has diarrhea. You can also give her One teaspoon of Pepto Bismol per 10 pounds of body weight for faster relief from her upset stomach.
I hope she is feeling better soon, Please let me know how she is doing.
This may simply be a case of Irritable Bowel Syndrome (IBS) since this is not the first occurrence I would start with the Pepto Bismol, and keep her from eating the grass. If she starts vomiting or has diarrhea start the bland diet again (If she isn't still on it).
I would recommend speaking with her vet about IBS and a more serious treatment plan for the long term.