Mama, please go to your OB or the nearest hospital/clinic as soon as you can, dizziness with darkening vision needs to be checked right away, especially when pregnant.
I know it's scary, first-time mom, pero better safe than sorry talaga. A few of us mums have experienced similar and it turned out okay, pero kailangan talaga ng professional check, could be something simple like low blood pressure or anemia, pero doc lang ang makakasure. About the folic acid dose, tanungin mo rin ang doctor when you go, kasi different pregnancies need different amounts. Fingers crossed for you, hoping it's nothing serious!