It could be an infection like a yeast infection (males get them too) or it could be an STD. STD'S usually show symptoms a few days after sex (not always) and infections happen within 1-2 days. I am no expert but this is something to be checked with one. Not even uploading a picture of it would someone be able to tell you what it was. Not even a doctor looking at it can assure you of what it could be. Getting tested is the only way to go. One note however is to check if the rash is red by blood vessels just like when your eyes get red, or is it a series of tiny little almost microscopic red dots. Trust me, you will need a magnifying glass to be able to tell. Red dots equal no good.