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, and my weight was staying the same, een when i was eating over 3,000 calories a day.   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 3,800.  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 1,600 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]

I think lots of activity can dehydrate you, which might help as a reason for the low weight :/