Why does my weight loss suddenly stop?

Im about 5'3 and 150 lbs. I have lost 10 pounds so far ( a pound a day) but now my weight has come to a hault. This has happened many times before, and I need to know why. Whenever I get down to the low 150's, I dont lose anymore weight, and I eat around 1200 calories ( sometimes less) and workout at least an hour a day. What am I doing wrong????