it could be any one of those things, or a combo of a few. the real questions are......are you limiting carbs (cutting perhaps)? do you feel your getting enough water? is it considerably hotter in your gym (or home)? are you recovering from a recent illness? etc etc.
if none of these apply to you (as in "same as usual" eating, lifting etc) then its unlikely they are the cause for the light-headed-ness (is that a word?).
on the off chance you are cutting and limiting carbs that could def be a reason for how you feel especially if your new to it. depending on what type diet/program your trying to follow (how many carbs and when) it usually doesnt take long to mentally/physically adapt to the new program and "feel better" while doing it. also, if the diet allows for it, a small amout of simple carbs pre-w/o can make a world of diff.
but that is all just a guess. the (other)real question to ask yourself is what is diff about your current w/os or diet from your usual routine
good luck