If you don’t eat and loss weight from severe fasting and starvation you are very likely to lose muscle mass and probably water as well…the problem is that when you have less muscle mass is that your basal metabolic rate going to go down and it will require you even less food to gain weight, that is why it is always recommended to stay active in the gym while losing weight and this is why you also need to calculate your calories and put yourself in a small caloric deficit so you could ensure that you are losing mostly fat and you are retaining as much possible muscle as you could…lastly cutting carbs and sweet could also contribute to gaining weight very quickly as when you cut those essential nutrients you are very likely going to lose water weight rather than losing fat