Periods changing with PCOS on birth control

Ok. So I have Polycystic Ovarian Syndrome. I take birth control so that I menstruate normally. I have always taken it regularly. Since I never had a period before taking birth control, the doctor told me to start taking it on Sunday. So that's what I've always done for 7 years. But my question has to do with the placebo pills and when I start my period. So I start taking that week of pills on Sunday. But lately, my period hasn't started until the next Sunday (maybe late Saturday). Then I start a new pack that same Sunday. Should I still be doing this or should I wait until my period is over? I know this probably has to do with the fact that I've gained weight and that's not good when you have polycystic ovaries.