She has had it for nearly 4 months, I have taken her to the vet twice, but both times they told me they didn't find anything unusual, but she keeps bringing up this liquid yellow vomit. I'm worried for her, normally she looks very tired during the day after she has had the problem... What does it mean, I have changed her food several times but the problem persists, and the vets (have tried two diferent ones) can't find anything. Any ideas?