Dreamer4ever
I'm so sorry this happened to you. What you need to remember however, is that this guy didn't "do this to you". I'm sure he didn't set out to hurt you and have a one-night stand. It it probably something that just happened - some guys (and some girls too) are just looking for a temporary fling and a little fun. He didn't mean it personally to you - it's just what he is looking for now.
I know it's a cliche', but the best thing you could do now is to move on. Stop following him, find a new friend, join a gym, anything that shows him you are just co-workers again. Good luck.
slow