Ive got three kids to two ex girlfriends. I may not be a "doctor" per se, but I am an expert when it comes to pregnancy scares.
yes, of course you could be pregnant. But it could also be:
* stress from thinking about it
* mind over body, from stressing about it too long
* an infection
* just sick from too much sex, that happened to me last week
* he may have gone deep (if you did doggy style or your legs went over his shoulders)
However, I dont know why girls dont just get the morning after pill. Crazy stuff