This is how opioids behave actually. They have a two-phase action at chemoreceptor trigger zone. At first they stimulate it and that's why people feel nauseous and vomit after the first doses (in medicine it's sometimes counteracted with neuroleptics like phenothiazine derivatives or butyrophenone derivatives), then they act the other way around and block it and it's often hard to force oneself to vomit. It resolves itself no matter what your reason of opioids intake is, be it for pain or in a recreational way.