question about working and gaining weight

Well I've posted quite a few things here about my struggle to gain weight and I know some people have suggested upping my calories constantly, and some have said my body is okay at the weight its at and not to force it.  [For those who havent' ready any my stuff before I joined here may 15th and I was 5'6" and 101 lbs at the time and I'm 20 years old].  I was pretty much sedentary for a while because I was only working part time 2 nights a week at a restaurant because I had finished school.   But 2 days ago I starting working at one of my old jobs which requires a lot of physical labor.  Its in a factory so I'm constantly lifting stuff that weighs up to 65 pounds and moving it around, and its no A/C so its really hot and humid inside so you sweat.  I'm on my feet all day from 6:30 am till 3 pm. 

The other day I weighed myself and I was up to 103 [Wednesday I thing it was?].  I didn't weigh myself yesterday, but I worked 10 hours at the factory.  Now today I worked at the factory for 8 hours, then I was at my night job for 5 1/2 hours where I wash dishes and bus tables all night, so again, no sitting.  I used that activity log thing to approximates how many calories I burned today, and it said like 2,200 [from working at walking to work and driving].  And I put in my food for the day and I was at about 3,700.  And I just weighed myself [after I had gotton home so I had just ate and hadn't gone to the bathroom yet] and my weight went down to 99 lbs! [at about 11pm]

Now My weight always fluctuates..usually from 100 to 103.  But I went down 4 pounds in barely 2 days.  Which doesn't sound like alot, but when I'm trying to get my weight into the 120's or 130's, its frustrating.  And you'd think my weight would have been higher since I had just eaten a cheeseburger around 6 pm, salmon and potato filling around 10 pm, and drank about 200 calories worth of ice tea].

So my question is, how many more calories should you eat compared to what you burn off.  If me eating 1,500 more calories than I burn makes me lose weight, approx. how much should atleast let me maintain then gain?  Or would I be better off trying to find another job which requires less manual labor? [problem is theirs not many jobs out there]

According to the burn meter here on CC, you burn about 3 times as much at work compared to what you'd burn just hanging around home(http://caloriecount.about.com/calories-burned -standing-moderate-heavy-a301). The fact that you're under 21 makes the CC calculators obsolete for you in terms of calculating your caloric needs. The BCM calculator (http://www.bcm.edu/cnrc/bodycomp/bmiz2.html) recommends 2190 calories for someone of your age, weight, and height to MAINTAIN their weight with ONE hour of exercise per day. For a sedentary person of your stats, it recommends 1560 calories. Therefore, the difference of these numbers (630) is about how many extra calories you'd need per hour of exercise. Your work, being an 8.5 hour shift, would probably be equivalent to 4 hours of exercise (maybe even more). In all honesty, I'd estimate your gaining needs to be 4500-5000 calories AT LEAST on the days that your work. I'm afraid to say that it's more likely to be about 5500-6000 calories, though. Your job sounds VERY strenuous, and that is a very low estimate. I eat about that much on days that I bike for a long period of time. It IS possible, but it takes A LOT of planning.

I hope that that helped...feel free to ask me any other questions that you may have!

Thanks so much for responding.  Wow thats alot haha, I guess I'm just gonna have to really work at increasing how much I eat and everything, especially on fridays when I work 13.5 hours.  6,000 calories is so much haha, but I will figure something out.