Basically your body is super tense up, heat beats faster, blood pressure rises, you do a lot of involuntary actions which produce stress and can feel painful. Then when you eventually vomit, your body relaxes and releases endorphins which give you a sense of euphoria and relief.
More complex explanation: [Here](https://www.reddit.com/r/NoStupidQuestions/comments/3w5w09/why_do_i_instantly_feel_so_much_better_after/cxtnvs3/?utm_source=share&utm_medium=ios_app&utm_name=ioscss&utm_content=1&utm_term=1&context=3)
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