wat im basically trying to ask is how am I supposed to gain weight while stop eating, but when I stop eating I just lose weight and get skinny

see I eat alot but dnt gain any weight. I think i've gained probably 1pound and a half the summer that past. and yes i know i've got a fast metabolism, which isn't really a bad thing. i jus cant explain how if u stop eating the weight will catch up, but wen i stop eating for a while i get very skinny like paris hilton skinny but with 5 more pounds added on.