I do not understand why this happened to me?

May be your scale broke down. 12 lbs. is way to much i have gained 6 lbs the most, but next day i am down 4 and 2 stay for a little while. i pee all night though. how tall are you?

Sounds like extreme sodium overload + normal weight fluctuation. Drink some water and just get back to your regular calorie goal. It'll be fine.
Edit to add: Don't weigh yourself at 1am after an extravagant meal. Just get back to your plan and weigh at your normal time.

I've had crazy fluctuations of 4-5 pounds from one day to the next but never anything close to 12 pounds. Always weigh yourself at the same time (first thing in the morning after you've done your morning duties is idea.) Don't upset yourself with a late night weigh in after a huge meal. A pint of fluid is a pound and if you've been drinking a lot of liquid, you could have extra "transient" weight there.

I'm not a nutritional expert in anyway, but I noticed you have high sodium counts on your diary. Sodium is not helping your cause ; ) Try to avoid processed or fast foods for a week and see if you notice a difference in your sodium levels and weight fluctuations. At the very least, a modest sodium intake will be better for your overall health.

That's an unusually large fluctuation. But the time span is far too short for it to be new body mass of any kind. So it has to be mostly water retention. Eating a lot more carbs than normal can do that temporarily. So can salt.

Don't weigh yourself for a day or two after a cheat meal. Your weight will come back down. Just give it time.

If your maintenance is 2k per day and you ate that plus 7,000 calories for that meal (way overestimating) then and only then would you see a true two lb gain. Just go back to normal and so will your weight. It just might take a week or two.

My guess is, you were low on Glycogen from a low carb diet and lot's of cardio. When you ate the carbs, your muscles were replenished with Glycogen, which attaches itself to water molecules. Go back to your regular eating and exercise routine and it will go away.

You didn't gain that much. It's likely the weight of the food, fluid retention, sodium, etc. Give it a couple of days. It'll be gone.

I'm 5'7. I ate the meal at 6PM. It's currently 1AM and I have yet to void or anything. Water retention?

Water retention + food weight.

Your weight will fluctuate immensely if you weigh yourself right after a cheat meal/day.

I had an entire XL Hawaiian pizza from Domino's before eating out at a brewery for my roommate's 21st birthday (I also had dessert). Long story short, I had a food baby that lasted well into the next day and ended up staying in bed due to the lack of energy lol.

Give yourself more time before weighing.

Well, first you're weighing your food. Second, you're going to be bloated as hell from all that salt. Give it a few days to digest and to get rid of the bloat. You'll be fine.

Weight goes up & down day to day & throughout the day. So take photos & measurements too.

Read this: http://www.myfitnesspal.com/topics/show/1080242-a-guide-to-get-you-started-on-your-path-to-sexypants

I'm 5'7. I ate the meal at 6PM. It's currently 1AM and I have yet to void or anything. Water retention?

Are you kidding? You ate a huge meal then weighed yourself a few hours later??? WHY???? And why are you asking such a daft question? Did you eat a bucket of pasta and drink a gallon of liquid.

Weigh yourself once a week at the same time. Stop obsessing!

Do you measure your waist? I wonder what a 12lb gain would add to your waist? I'm guessing 3 to 4 inches.

My guess is, you were low on Glycogen from a low carb diet and lot's of cardio. When you ate the carbs, your muscles were replenished with Glycogen, which attaches itself to water molecules.

^^^^ I think it's a combination of this plus sodium water weight, and probably your scale is dodgy as well. Maybe throw in some TOM water weight gain as well....

One things for sure, you didn't gain 12lb of fat overnight.

If you're eating too little and doing too much cardio that's counterproductive in the long term, so you might want to think about eating more and/or doing less cardio.