Is it normal for your weight to go up and down during a diet?

Hi, recently I have taken up this new diet and it consists of not eating after 8 and no carbs or junk food after 4. After two weeks ive managed to come down from 80 kgs to 76.5, but this morning when i weighed myself i was 77kgs. I dont understand as ive been really strict and been doing excersize at night. Any advice would be appreciated. Thanks