It will just be dehydration. You will put the weight back on as soon as you start eating and hydrating properly.
Interesting point about not gaining weight, though. I don't buy into the ectomorph / hard gainer stuff (although I always used to think of my self as an ectomorph), but I'm consuming 4000 calories a day and have lost 3lbs in two weeks. I don't have an answer for that one; just empathising.