Why not send him an e-mail and start some chit chat? I've been in the same boat (Wrt a co-worker) and unfortunately you need to communicate else it will become blatantly obvious to all other co-workers that there's something wrong.

Most guys do tend to handle the whole concept of one-night-stands a whole lot easier than most girls.
Unfortunately you also need to take responsibility of what happened - you knew you weren't in a committed relationship and therefore cannot expect him to treat you like a girlfriend.

One-night-stands are very complicated, as you cannot bring emotions with; and especially as a girl, this is hard.

I'm so sorry! (((HUGS)))