Is weight gain a day behind?

I have been losing weight steadily for about a month and eating very well. On Sunday I decided to jump off the wagon for one day and had a super greasy bacon cheeseburger and fries at Ruby Tuesday, and that was all I had all day. I was surprised to wake up on Monday and have lost another pound. On Monday I went back to salads and grilled chicken and went to the gym. I wake up today (Tuesday) and after a super healthy day now I've gained a pound. My coworker says it's because it takes awhile for food to break down so this is what's catching up to me from Sunday. Is that possible/true?? Thanks!!