I take multivitamins but now my periods have stopped completely. it's been 4 months around the same time i started these vitamins. could it be these?

Doubtful: Unless they have some type of estrogen/progesterone activity. It is more likely that something else is causing this. I suggest you see you physician for an evaluation.

Doubt it: How old are you? This could be the change or it could also be pregnancy. Have you had a pregnancy test? Other causes for the absence of periods could be ovarian cysts, stress, thyroid disorders and pituitary disorders. Please check with your OB .