why did my menstrual cycle get longer?

I started having periods when I was 12 and a half. they were always regular and on time like clockwork, every 28 days I would get my period. I never had to guess when it would come, i would know exactly when to expect it. now, at 17, my period has been a little irregular over the course of about a year. it has been slightly irregular, sometimes they would come late, like 2-5 days late. One period was 17 days later than the expected arrival! but now my average menstrual cycle is about 32-33 days. what may have caused this change?? is it something to be worried about and talk to a doctor? please help. thanks !!