D.P.
I'm pretty regular on a 29 day cycle but I had three months in a row here recently that I was anywhere from 7 to 10 days late each time. Weird -- no reasons...my obgyn wasn't concerned, she just said to keep track of it and if it persists, then we'd check my thyroid. This last period was only 3 days late (which isn't odd for me if I'm stressed).
I'm also one of those girls whose period regulates with other women. There is one girl here at the office that for some reason my cycle linked to hers. Sometimes our bodies just do weird things!