I have pcos, on metformin and control for almost a year and a half now, for a few months i had normal mc, recently i've been a few days late, why?

Normal variation: If your periods are only a few days late this probably represents nothing more than the normal variation in the time it takes eggs to mature and your periods are still essentially regular.It could also just be the result of being on birth control pills for awhile which can make periods shorter and thus make them start later. You are to be commended for such good control of your periods.