The most obvious cause of light-headedness is insufficient oxygen getting to the brain. That could be because of low blood pressure, blood loss (as with a woman with heavy periods), internal bleeding (as when I bruised my spleen), dehydration, blood disorders, etc.
However it's just as likely (or probably more so) that you've got a minor head cold and simply haven't noticed (or yet developed) any other symptoms.
It's one of those 'if in doubt, get it checked out' things but there's probably nothing much to worry about.
This
might be relevant:
http://www.nhs.uk/conditions/dizziness/pages/introduction.aspx