My dog keeps eating grass and vomiting foamy liquid!!?

my dog is 9 months. She has been vomiting like this almost every day for a few months. I took her to the vet, but the goddamned vet had to break out a textbook to tell me that she didnt know, it could be a bunch of stuff. She eats food she's never had a problem with. When having to switch her diet in the past, I gave her plenty of time to adjust. No matter what, she still stands outside every morning and eats grass for like 20 minutes, then usually throws up. This has been going on for too long and I dont have the money to go to 2 ******* vets. Please help!