If your period has been regular, it’s unlikely your period will start when you are not expecting it. However, some medical conditions like polycystic ovaries, severe prolonged stress, excessive loss of weight may push your hormones to the extreme. This may cause your menses to come unexpectedly.
In women after menarche, (that is after begins for the first time), it’s difficult to determine your next period. This is because hormones are still unstable and your body is still trying to learn menstruation.
This infographic from Medplux eases your stress and provides early period symptoms.