Is it bad to lose 1 pound a day?

I started at around 151 lbs on Sunday, this morning (Thursday), I weighed in at 146.8. That's like more than 1 lb. on average a day. If I only do this "extreme" diet for another week or so, is it a big deal?...In case you want to know, I eat around 900 cals a day (including a protein shake), and I burn 600 cals on a stepper along with weight lifting daily.