1. How many hours do you exercise a week? You might eat like... 900 calories but if you only burn 800 calories, you're still gaining weight. To lose weight, there needs to be a deficit like rndmguy said.
2. If you really do eat 900 calories, then that's not good. Because it will make your metabolism slower. A normal body takes 1200-1500 a day and any lesser would probably lower your metabolism, maker it harder to burn calories.
3. The weight of the body does fluctuate, so if you're going up, then down a bit, then up a bit, that's normal. What counts is the bigger picture: in the LONG term are you losing weight or gaining weight?