I've lost alot of weight recently, about 40 lbs over the last 3-4 motnhs.. and its been good im my ideal weight, and im fit... but i really ate a bunch today for the first time in ages, and i feel really bloated, just wondering if ive actually gained any weight? or would weight gain be a process over time? some one put my mind to rest!

You can gain 'weight' in seconds. But it takes time to gain 'fat'

Remember that they are different cos its really important. Yes you will have gained some fat, but not a huge amount from one day.

but it would have take about 3500 cals for me to have gained 1lb?

ive probably only had about 2000 cals today... the most in aaaaaaages. if i weigh myself tomorrow will i be the same weight as i was yesterday? i dont have scales at home.

whats your age, weight and hight?

1g of fat is 1kcal, then it depends from person to person how of it the body takes in.

1g of fat is 9kcal, not 1

ofc, mistype

The more carbs you take in, the more water you'll retain. Same thing with salts, so no your weight won't be the same; especially if you've taken in an abnormal amount of carbs than you're used to.