Optivite Side Effects

Hi, I've been taking a supplement called, "Optivite P.M.T" to control my horrible PMS symptoms. I've only been taking one or 2 tabs a day (as opposed to the full dosage of 6 to test it out). My period seems to have come about a week early, only after 18 days and my cycle on the average is usually 25-26 days. I am still on my 12th day of bleeding.  Is this a common side effect of the medicine? In addition to Optivite, I take supplements of magnesium malate and riboflavin to control my migraines that come during this time.