What Causes Irregular And Painful Menstruation?

my last period was on august 28 and lasted 3 days. i started a new birth control after that and was told to skip placebo pills so i would only get my period every 4 months. i didn't get my period in september, but oct 5 i got it and was unprepared and have had it since, now 12 days. i have shooting pains lower than normal cramps. is this normal?