Are you on any birth control pills?
Easily, particularly if a stressful event has happened to you in the past couple of months or you are around a lot of different women than you were previously.
I believe that the stressful even adds a bunch of hormones that can trick your body into changing or attempting to change its patterns a bit. (It doesn't always.)
A change in the social group of women around you can cause this because oddly enough women give off hormones and somewhat synchronise. So your body clock could be shifting due to that.