Why the Sudden Lost of Interest?

No way to definitely tell without knowing either of you, but I think he might have the wrong impression of you. You were very stand offish, but then all of the sudden a pionate kiss? Maybe he's not taking it seriously that your interested, but just there because he wants to be with you, not that you want it. Then again, he could have lost interest in that he was looking for something different in you, not that it's good or bad, but you are not a good match after all. Who really knows? I say, act the same way as you did before when he was interested like ditch/ ignore him etc., and see what happens, and go from there.