It certainly isn't unheard of. Are you excercising aswell? Are you weighing yourself at the same time of day (ie. first thing in the morning). Is your caloric intake correct?
When I first started out, I lost around 4lbs a week for the first couple of weeks but, I was doing crazy amounts of cardio aswell.
I wouldn't worry too much if your diet and training are in check, it will probably level off before long.