At less than 600 calories/day, you aren't eating enough. Your body needs so many calories just to function and if you don't provide enough, it will hang onto current fat stores. Typically, one should never go below 1000-1200 calories/day, but your actual calorie needs must be calculated, based on your height, weight, age and activity level.
What kind of exercise do you do?
I agree 100% with Barb. Anything below that 1200 range is not only dangerous, but will most likely result in regaining any weight lost at an even faster rate.
Rather than crashing, try to set a realistic time frame for your self. For starters, losing 1-2 lbs a week would be a healthy rate for your personal measurements.