What Causes Late Periods?

HI, I m a 16 year old female and my period is about 2 days late. My last period started on the 14th of July and on the 16th the month before, which is quite early for my normal schedule (usually start on the 20th). I am going to back to school soon which is a bit stressful. I had protected intercourse on July 22 so the fact that my period is late is a bit frightening. I m positive that the condom did not tear. Could there be another possibility as to why my period is late?