I vomited a couple of days ago and one side of my lower back has been killing me since then. could i have injured myself throwing up?

It is likely that: you pulled or strained a muscle in your back when you were vomiting. Your symptoms could also indicate urinary tract or kidney infection. Try using a heating pad for a few hours to see if it helps but if it does not go away and/or you develop fever, chills or urinary symptoms you would benefit from evaluation and should see your health care provider.