Is my contraceptive pill less effective if my menstruation is irregular?

If you haven't forgotten to take your pill, it should be just as effective. You should continue to take your pill even if you have your period. Several factors can influence the onset of your period, so the fact that your period has been "off" for 2 cycles in a row is more inconvenient than worrying. It would be best to contact your gynecologist/doctor to advise them. Your family pharmacist is the best person to answer your questions about your prescription medications.