Why haven't I gotten my period?

My last period started June 2nd, and lasted until Monday/Tuesday which is a little earlier than usual. I get mine every 3 weeks and it's usually right on the day that the 3 weeks is over. While i was on my period in June, my boyfriend and i messed around but no sex. My period is now late by a few days. Why could this be? I've been under a lot of emotional stress too, and I don't feel sick or anything. So, what could be wrong?