Well, tough question.
Eating grass, in itself, would suggest there is some degree of stomach upset. So, possible causes could include:
- Dietary indiscretion (eating something you do not know about)
- Stomach or intestinal foreign body
- Something that was given to them (new food, new treat, human food)
- Intestinal parasitism (not just the worms, but the microscopic bugs like Giardia and Coccidia). They can cause vomiting also, not just diarrhea
- Toxin exposure would be less likely, unless you have reason to suspect exposure to a chemical.
Episodes of shivers:
That is a very real concern to suggest bad nausea and possible abdominal pain. If any type of shivering or tremoring persists, you really will want not consider an ER vet visit.
Possible testing may include abdominal x-rays to look for any foreign material ingestion.
To help settle the stomach you could give a 1/4 tablet of Pepcid A.C. 10mg antacid, or a ½ tsp of pepto-bismol.
Even if no vomiting or diarrhea, currently, still a seirous concern.