Why do people say that you can gain weight from not eating?

ok im 16 and even though i don't look overweight i am, and it's not like im delusional i really am when i step on the scale. My weight is distributed evenly all over my body so im "thick". So yea, i've been eating right and exercising for a really long time trying to lose the weight so i've decided to refrain from eating for 7 days (I'll still drink throughout the day) and then i've decided i'll become a vegetarian after wards. So yeah everyone keeps saying that i could actually gain weight.... how is that possible????