What Causes Abdominal Discomfort,bloating And Vomiting?

I have abdominal discomfort and bloating which I can partially control with antacid tablets. Twice it has become so severe that I have vomited. In one case the vomit was copious, all liquid, brown and very bitter. In the other it was rather scant, light brown and extremely acidic. I have had two sessions of this in the last year but no idea what causes them.