You may be dehydrated try raising your water intake and rest, maybe your body isn't processing your pregnancy to well and thats just one of your affects that this pregnancy will give you. Hope you feel better!
I had the same thing lightheaded dizzy it even got so bad I started seeing light spots float in the air or loose sight lots of water take it slow stay out of heat and eat when you get hungry it will help just talked to my dr yesterday about it good luck hope it helps you should also call in because they told me if it's really bad go to l&d