I had my period last week. my nipples are darker and i have blue veins on my breast. i've been dizzy and light headed all day today? what could it be?

Anemia: Menorrhalgia, or dysfunctional uterine bleed can lead to iron deficiency anemia, producing orthostatic hypotension, dizziness, lightheadedness, malaise, fatigue, craving for ice, or pagophagia, a variant form of pica. If these symptoms persist, you may want to followup with with your physician to obtain additional blood tests and possibly iron supplementation if needed.

Pregnant/dehydration: First step would be a pregnancy test, second to drink lots of fluids and some extra salt. If it keeps up, see a clinic.