I have a 2 year old Border Collie mix. For about the past two months, about once a week, she will vomit, yellow stomach acid in the morning about 5:30AM. She did it yesterday and today. Should I be concerned? We feed her at 8AM and 5PM and there is never food in the vomit. There has been grass a few times but not every time. She does not appear to be losing weight either. Do you have any idea what might be wrong?