If you lose weight that fast you are definitely doing it in an unhealthy way. I'm vegan so i'm all for eating right and exercising to loose weight. If you do that you will weigh what you are supposed to weigh.
For the flat belly I would recommend leg raise and crunches and reduce ur sodium intake because too much salt will make your body retain water and for the weight lose try to eliminate any fast food, fryed food and junk food do at least 30 minutes of cardio 5 times a week and definitely reduce the sugar u take in as sugar turns into fat and DEFINITLY don't eat any thing with high fructose corn syrup and u should be good just don't cheat yourself and work for it just look at me :) hoped this helped
Not safe to do. If you really have to you can try the new diet where a doctor jams a tube up your nose and you don't eat but get fed 800 calories a day through the tube in your nose. Best part is how sexy you will look with a tube up your nose. Just exercise and lose what you can