What Causes A Delayed Menstrual Period?

Over the summer, around the last weekend of june, I had unprotected sex. The next day within 24 hours I took Plan B (the day after pill). Two weeks later I had my period which was two weeks before I was supposed to get it (not more or less than usual). My normal menstruation cycle is pretty regular. But since then, I have not had my period (It's been approximately 3 months). However, in August, I had travelled overseas for about 5 weeks, changing my diet, environment and stress level. I have been under stress lately but nothing more than what I would usually have in school. Might I be pregnant? What's causing the delay?