One or even two days late is not abnormal. Mine never comes on time like, ever. If yours is ALWAYS on the same day, regardless of the situation, even then I'd wait a few days before you get nervous. Although a quick pregnancy test would solve your worries.
Personally, your period's arrival can be affected by factors like stress, travel, sickness, and other things in your life.
You sound like a very responsible young woman. Hope all is well, and that you don't have to be a mother *just* yet.