Question!

So I'm currently waiting for AF to start... According to a positive opk I am 5 days late... According to to glow in due on Sunday and usual cycle would say I'm starting today.. Now I was taking prenatal this cycle and I noticed ever time a take any kind of supplement or multivitamins it throws off my cycle!! I just don't know what the answer is anymore!! And I know I'm not pregnant (took a test today BFN!)