More than likely it is something hormonal. It could be anything from stress, depression, or even eating habits changing. If nothing abnormal has happened, and you think that you are pregnant then do a pregnancy test.
Three days early is not outside the realm of normal menstration. Having a period only last 2 days is a little stranger. You're probably not pregnant but are just having a hormonal fluxuation. Take a home pregnancy test to be sure.
More than likely, if you had more than minor spotting you are not pregnant. Changes in diet, exercise, weight loss, or gain, are only a few things that can alter your cycle.