Am I correct in presuming this is not a prescription medication? (Rhetorical question...) Probably what's happening is simply that you're straining various muscles in the process of vomiting. The headaches & lightheadedness would also be in line with vomiting over a period of days. And then, of course, there's the effects of the drug itself... whatever that is. From what you wrote, it sounds like you probably need to stop taking this drug... whatever it is...