I've had dizziness throughout this preg (now 29 wks). I have been taking prenatal supplements. Bloods results were lost 
so my gp suggested retesting at 25 weeks which showed low ferritin (should be 20-200, mine was 16) and I was put on a course of ferrous fumerate, 240mg twice a day (prenatal tablets have about 20mg). 3 weeks on there is a bit of improvement to 25 but now my RBC count has dropped below acceptable levels.
Anyone gone through similar and what did you do? I'm on so much iron I can't imagine I need to take more?!