What makes period irregular every month?

I was just curious to know if anyone had an extremely irregular period. Or any answers. It had been regular almost since I first started my MC at 28 days apart. Now over the last 7 months I always chart on my glow but it never seems to match the start up date. It has been anywhere from 6-7 days early from glows start date, to 5 days after glows supposed start date? Varying between months. Any suggestions. I am not on birth control, within a healthy weight range and regular everyday life style? So any other factors?

Thanks in advance!