Two possibilities: either you are pregnant, or your ovulation calendar was off and you didnt' ovulate when you thought you did. It's ovulation that triggers the luteal phase (time between ovulation and your period) and the luteal phase tends to be a pretty consistant length for each individual woman. However, ovulation can vary based on stress, illness, and just random factors. I'm not sure what kind of calendar you are using, but I recommend the book Taking Charge of Your Fertility if you want to find out a way to pinpoint your own body's fertility signs- basing it on past cycles isn't as reliable. HTH!
at 11:00 PM on Jun. 7, 2011