I would not lose sleep over it unless the stopper itself touched something dirty. If it was only open for a moment you are likely okay, just clean the top thoroughly with alcohol before drawing from it. Subcutaneous shots already carry low infection odds when basic cleanliness is followed. That matches my comfort level, though others may see it differently.
After months of injections without problems I managed to pop the rubber stopper off my BAC water vial this morning by applying too much pressure. I went ahead and reconstituted several vials anyway, but then an AI response made me second-guess the decision because of possible contamination from even brief air exposure. I also recalled that a doctor once supplied plain saline for reconstitution, which has no alcohol, and I stored the mixed vial in the fridge for a week. On top of that, the usual practice of pushing air into the vial to ease drawing liquid means we are already introducing room air every time.