Not feeling good

I don't know if I am just tired, if there are otherwise undetected gases floating about this site I'm working on or if I am unwell, but I am feeling incredibly light-headed at the moment. I've not eaten less than I normally would and I don't feel hungry, so it can't be that! I do recall that when I was working last weekend (on day shifts then) I felt a little light-headed for a short time while in the control room, and felt like I was possibly hallucinating slightly. Well, while I was just on my one-and-only patrol of the night just now I felt like I had to be really careful not to collapse. Not a nice feeling at all. It was made worse when I had to look under a toilet door that was locked to make sure that no one was stuck inside as that made me even more dizzy. It is really strange and I will have to keep an eye on this in case it gets any worse. I certainly can't work on a building site (as this site is at the moment) if this does get worse. I'll keep you posted.