What Could Cause Missed Periods?

About a month ago i gave my boyfriend an hj. His semen got on my hands and i washed them an went to the bathroom. A second time, i didn't think any got on my hands so i just went to the bathroom instead of washing. Now i haven't had my monthly period. I have been having a lot of stress lately, could this be the cause of my missed period? and i am athletic as well and 15 years old. Could i be pregnant?