Could be dehydration, could be iron deficiency, maybe your stood up too fast. It could be anything.
I'd say call your OB and let them know. They may advise you to take an iron supplement.
Good luck dizzy mama! =3
Try more water sit at the edge of the bes before standing and keep a small snack of protein like nuts in your bag at all times... I was the exact same way in the beginning... it *****!!! Feel better hun!!!