Not tying to rain on your parade, and you must have been pumped shooting your first buck, but a shoulder hit you should wait at least an hour if not more. If he was quarting a little towards you you might have hit a lung. Odds are where you found your arrow is where he stopped and possibly pulled it out. You might have bumped him if you did not wait long enough. Tall grass is a good place for a wounded deer to lie down.