Take measurements of your arms, chest, and waist. 1 pound of fat and one pound of muscle weight the same, but muscle takes up 1/3 the room that the fat does. You could be losing inches instead of pounds.
I wouldn't go under 1000 calories in a day. I would try adding some more exercise to your day (your body adapts to what you do quickly, so it's already used to your work shift and the stairs). Remember to drink plenty of water as well, it helps flush out all the stuff you don't need.