Actually that might be difficult to determine. Do you know when you ovulate? Lets say that you ovulate 14 days after the first day of your period. Sperm has been known to live for 72 hours possibly even longer. Meaning you could've ovulated right after you had sex with your boyfriend and you may of still be ovualting with your one night stand. But right now you're not sure that your pregnant, do you have any other symptoms? Right now if you were pregnant you wouldn't be very far along, maybe a week or two. Do you have sore breasts? When is your period due?