Trixie had a similar problem not long ago...same symptoms you described minus the vomiting. All her parasite tests and blood work came back negative and everything. She had to eat boiled chicken and rice for 2 weeks and take an anti-inflamatory pill. She cleared right up, no problems since.
Our (regular) vet said that there are alot of things she could have eaten that could have caused it. (After talking with the emergency vet, they both agreed whatever it was had caused intestinal inflaming and upset.) She said that one of the most common causes is beetles and particularly stink bugs!
She said most hard-shell beetles (even ssmall ones) have an enzyme in their stomachs for digestive purposes that is toxic (toxic, not deadly) to dogs and will cause this problem. Of course, there could very easily have been something else, but I would have never thought twice about Trixie attacking a beetle if she saw one.
