I have one flat, round, small red circle on my left shoulder. doesnt itch or hurt. i just noticed it there one day.

Petechiae: I cannot confirm anything without examination but from your symptoms it sounds like this may be a petechiae. These form from small blood vessels breaking under the skin. this should resolve in a couple days and if not you should seek further evaluation from a physician to r/o more serious causes. These can result from viral illness or microtrauma, they could be benign but worth checking out.