As others have indicated, without medical supervision, folks shouldn't eat less than 1200 calories per day. That is about the right number of calories to maintain a little less than 120 pounds of body weight, so if that's a bit too high for you, physical activity should make up the difference.
The phenomenon Jynohn mentioned is interesting: It's actually more a matter of not eating enough protein, than not eating enough calories. If you increase calorie consumption, then generally your weight will increase, all other factors held constant. Protein intake, for someone who is protein deprived, however, sometimes increases metabolism, which accounts for the phenomenon. It is temporary though. Eventually, the body adapts, and you're back to plain-ol' calories in versus calories out.