I've been having weird cycles like this since november. I'd have a full 6 day period, a week off, and then another full period. Went on for a couple months. So in essence, my cycles were 14 days-ish. Turns out I wasn't ovulating, and the erratic ups and downs of estrogen would make my lining shed at random intervals.
My OB when I went in for it at first did some blood workups and a vaginal ultrasound to look for cysts or polyps or something, but that all came back normal, I wasn't even anemic or anything. Since we were actively TTC, I charted until last month to gather "evidence" that I wasn't ovulating, and we finally made the decision to go on clomid.
So I guess moral of the story is: Get yourself checked out to rule out "mechanical" problems, endometriosis, etc. If you're not TTC, I suppose you could regulate it with the pill, or just accept it so long as you're not anemic and all that. Or, if you ARE TTC, you may have a deeper observational period ahead of you.
Mama to twin boys, Oct-'09 and baby girl, Apri-'12!