Let’s think of this scenario. If it’s possible to gain even 1lb of fat on 1 day, then it’s possible to gain 1lb on another day, and so on.
If you could do this, then in theory you could gain 365lbs in a year. I guarantee that never happened in history.
There are many factors that could have affected your weight.
It could be the pizza treat meal the night before. The consumption of alcohol also the night before. Water retention due to eating more carbs, menstrual cycle or even training legs the day before. When you train legs, it requires more blood volume. More blood volume means more water.
We’ve all been there. When the scale goes up overnight, you feel discouraged. But it’s just temporary. It’s should not affect your long term goals and progress.
What I encourage my trainees is to weigh yourself 3x to 4x a week, then take the weekly average. The weekly average will wash out any 1 day of surprising high or low weight. This is a much better reflection of your actual weight in time.
But it’s also important to know that being in a caloric surplus for a prolong period of time will ultimately results in weigh gain.