Sometimes a new website shoots up in high search results (artificial boost) but settles down to its actual position a little later...or maybe Google is filtering out spam. Did you indulge in something like black-hat?
I tried locating other possible reasons too but then thought Google�s Matt Cutts can explain it better. Here�s the man himself helping you:
http://www.youtube.com/watch?v=gx04qNrCbnk