How much weight will i gain?

i havnt been eating any fatty foods in 4-5 months. ive been on a 1200-1500 calorie diet for 4-5 months but today i had hungry jacks, chicken burger, chips, coke and a sundae. im 14 years old and was forced to eat hungry jacks today by my mother because i did something stupid. i took 15-20 laxatives on an empty stomach. how much weight do you think i would gain from this?