Period start date changing HELP!!!

I have PCOS and was told that birth control pills would be the only form of treatment. I’ve been on the pills for two years and have always started my period the second Monday out of the week. The past couple of months I’ve been starting on Tuesday. I should have started by today, I have yet to start. I’m still getting all my symptoms for my period. Just confused why my period start date has been pushed back. Any suggestions or comments?