You have 72 hours after the unprotected sex to use plan b. And your options are:
1, Get tested now.
2. Go through your Facebook/emails and try to figure out exactly the day you're cycle began,
2a) and then do some independant study of menstrual cycles to try and figure out if you had sex while you were ovulating.
2b) and wait until your 35th day, and then get yourself tested (35th day is normally the longest length of a regular menstrual cycle for teens/young adults. However this number can increase if due to medical conditions such as an infection or the like)