Think about this:
To lose one pound of fat, you need to have a deficit of 3500 calories. You need to burn 3500 calories more than you consume.
To GAIN one pound of fat (not water, muscle, anything else, just fat!), you need to CONSUME 3500 calories more than you burn.
So the question is this:
Let's say you burn maybe 2500 calories in a day. I don't know your weight, so I am purely guessing here. 5 lbs of fat is 17,500 calories.
So, to gain 5 lbs of fat in one day, you need to consume something like 20,000 calories IN ONE DAY. Now, did you suddenly forget that you ate and drank 20,000 calories?
I doubt it.
More likely, you just made bad choices, went a little bit over, had salty foods (maybe Chinese or sushi, or chips?), and you gained some temporary water.
Take it easy. Keep doing good and it'll be gone in a few days. There is no such thing as a 5 lbs fat gain in a day. It doesn't exist.
Good news, isn't it?
"Losing weight is never about eating as little as possible"
- Kingkeld.
"You wouldn't worry so much about what others think of you if you realized how seldom they do.
― Eleanor Roosevelt
"Do. Or do not. There is no trying."
- Master Yoda.
From October 4th 2010 to April 3rd 2012 I lost half my body weight - 80 kilos/170 lbs. Since then, I have had two cosmetic surgeries to remove excess skin. I have now quadrupled my strength, gained several kilos in muscle mass, and today I focus on building muscle, optimizing my diet, and living healthy. I am stronger, healthier, thinner, happier!