I'm not sure about the effects of BC, but irregular periods are something to go see your doctor about. What type of BC were you on? I don't have any experience except for Depo Provera and I took that for about a year 12 years ago. Hated it and it took months for my periods to get back on track.
My advice would be to talk to your doctor and determine the cause for your irregular periods. My guess is that if you are having two a month, then you are not ovulating regularly.
~Jenn~ Mom to Ethan - my heart surgery survivor - born 6/29/09