Nuts mixture / healthy diet or?

actually I'm on a diet, I don't eat food with a lot of calories and don't really eat much and I lost 3 kilos in 7 days. today I decided to eat only fruit and a nut mixture like almonds, cashews, Brazil nuts etc. so I didn't eat more than 900 calories, but I gained 1 kilo. I feel really full and my stomach looks a lot bigger too, and it really hurts. Why did that happen?