not getting period every month

I've had a period for 9 years and up until last year they were always on time, 28 days in between and lasted 5-6 days. last year I was on birth control that I got at planned parenthood and took it for 4 months. I only had 1 period during that time and always thought I was pregnant. I stopped taking it over a year ago and still have problems. I get a period every 1 and a half-2 months now. does this happen to anyone else? did the pill mess me up?