Nothing really. How old are you? If you've only recently started having your period it takes a long time to become regular. Outside influances are also cause for your period to come early or to stay...
Most likely that your just having an irregular cycle. It can happen for many reasons including stress, recent illness, hormonal changes, and exercise or diet changes. If it continues to be a problem...
Rarely are women so well timed that periods always start at exactly the same time. Three days early means three days early, nothing more, nothing less.
