Every third period

rainlover • just turned 21 last October and really trying to make myself healthy and getting ready to get pregnant. I have pcos and in may it will be two years off birth control so hopefully I get there soon
So every third period I have is either very late or I just miss it completely and I get it normally the next month everyone have any ideas that can cause this or idk it doesn't bug me or make me think something is wrong with me just wanted some ideas it just might be the way I am lol does throw a good scare of being pregnant but then I remember its the third period lol