What is causing my dog to occasionally vomit bile?

This past week my dog has been vomiting occasionally. It has happened three times and the first two times she vomited early in the morning before breakfast and the third time she vomited late in the day before dinner. Every time she threw up a hy bile. I am planning to take her to the vet this week but I figured I would see if anyone had any idea. She seems very happy and her appetite is good. She has always been a fairly picky eater about her dog food so when she occasionally does not to eat it doesn't really concern me. I know pancreatitis has similar symptoms but she is acting totally normal other than the occasional bile vomiting. She never throws up food. Could she be doing this because she needs more food or more meals? I feed her twice a day.