i have very similar problems, they have put it down to a combination of low blood pressure and my new heart murmur
I would speak to a mw about it, but I wouldnt get yourself worried about it (that will only make you feel worse)
I have been told to just take it easy as much as possible, if i go out I have to have lots of breaks, always have water with me, things like that
the other thing is, mine has been worse when I have gotten ill as well (which seems to be happening a lot) so possibly you may have a bug aswell, and your body is just struggling to cope?