Taking PM will likely affect your cycle, no matter how regular or irregular you are. How it affects you depends on how you take it. You should consult your doctor about this. If he does not know what PM is, just tell him it is a strong phyto-estrogen.
Please, see this article on irregular periods:
http://www.webmd.com/women/tc/missed-or-irregular-periods-topic-overview
Please, see this article on irregular periods:
http://www.webmd.com/women/tc/missed-or-irregular-periods-topic-overview