What Causes Irregular Menstrual Periods?

Hi, I am a 46 yr old female, general health very good en exercise regularly. I have two children and not been on any birth control for the last 6 yrs as my husband had a vasectomy. My last checkup about 9mnths ago and PAP was normal. My periods have been always on a normal 28 day schedule. The last 3 however it seemed to start one week early and after one day it stops and come on a couple of days later. Would appreciate your advice.