Might be low blood sugar? I had lots of dizzy spells but it was in the first trimester. I found that ginger ale helped me because of the sugar. I would call your doctors office and ask a nurse about it. Also, make sure you are eating/snacking enough during the day.
Do you have any other symptoms? Perhaps sinus pressure? Or ear pain? I know when my sinuses get very congested I feel really loopy, same from my ears. I used to suffer vertigo. Make sure your sinuses are clear. Try a saline spray, and for your ears, sit up straight and look forward. Lower your chin slowly to your chest then slowly rock your head from side to side a few times. Its supposed to help with vertigo. Google it. There's lots of info explaining why it happens. If that doesn't help perhaps the baby is pushing on a nerve? May want to contact your doctor. Last thing you want is to fall from dizziness.