Even if you did eat 3,500 extra calories, the human body can not metabolise food that fast i.e. convert food to fat. If you "gain" weight, it's because of the physical weight of the food.
Ever notice you gain weight during your menstrual cycle ? Is that a fat gain ? Nope, it's nothing more than a temporary water weight gain that passes in a few days. If you gain weight overnight, it's because of water retention or the physical weight of all the food you ate that day.
Most people don't realize that it takes time for the human body to metabolize food into fat. It doesn't happen overnight. So, if you gain weight, it's a temporary gain that will pass in a few days. Also, not to get TMI, but if you don't have a bowel movement in the morning, that too will effect the reading of the scale. Not only does food have weight, but so do your body's waste products. So, if you don't do a poop in the morning, that can cause the scale to go up.
Try not to obsess about what the scale says. Yes, we do need to be mindful of what we eat, but people don't gain a pound of fat in a day. it's continuously eating excess food day after day after day that causes the weight to pack on. One day of overeating won't hurt, but you don't want to do it every day.