Period is getting later and later every month

Okay, when I first got my period some 13 years ago, I never really knew to pay attention to the pattern, but I would say it was about 30-31 days.  When I got married and went on the pill, it was, of course, a steady 28 days.  I went off the pill almost 2 years ago (after being ON for 2 years) and it stayed at 28 days until roughly 6 months ago.  My period has gone from 28 days, to 30, 31, 33, and we are currently sitting at 38 right now.  I understand that the average irregular period is about twice a year, but this has been consistant for 6 months straight.  Pregnancy tests say NEGATIVE (usually taken 3 days after 28 day mark) and period does eventually come.  I was wondering if anyone had any idea as to what it might possibly be before I go running to a doctor.