What Does Dizzy Spells And Brown Spotting In Woman With Iron Deficiency?

Hi I have had the merina for about 6 months with no issues. I have always had irregular bleeding from 3 months straight to nothing at all for 3 months. I have fallen pregnant twice on the pill and didn't know with my first til I was 3 months due to my irregular bleeding. In the last 3 weeks I am regularly getting dizzy spells, blurry vision, cramping and brown spotting. I am iron deficient and suffer low blood pressure when pregnant only. I had a low hemoglobin and needed a blood transfusion with my second bub . Basically I just wanted to know what your thoughts may be to what might be happening?