My cat is vomiting food despite hairball control?

We obtained my long haired cat from a shelter and since we got her she has been vomiting her food and hairballs. We feed her hairball control food. Well recently, its gotten to be where she is hardly eating but she is begging for food. We changed her to a different hairball control flavor and she still isn't eating, but yet she is vomiting. If someone can please help me with this problem it would be greatly appreciated. I don't know what to do, and were about fed up