Period- dizzy !!

Hi, I'm on yasmin, this will be my 4th month on it. 
So I went on it on Thursday- from Friday night I had light pink blood like water, through until Sunday then it got  bright red abd very clotty. But since Thursday iv been so lightheaded and dizzy, near passing out a few times. This has never happened to me before and my period is lighter than ever with hardly any cramps. 
Why am I getting very dizzy anyone have any ideas?