It's your BP. I am an APRN in a cardiology practice and we see a large population of dizzy/lightheaded and syncope patients - yours is classic for low BP...especially on this diet. Add more salt. Also, if you *chug* a bottle of water before even stepping out of the bed, this will raise your BP by 10-15 points for 2 hours or so. But, you have to *chug*, not sip