@naria,
The odds are probably against it, but I'm no expert in the ovulation/fertility cycle.
What I am an expert at is enjoying spontaneous, unprotected sex for a little while one night, then spending the next few weeks sweating bullets and losing sleep, wondering whether or not I'd impregnated her. Not worth the anxiety, by a long shot. I recommend doing something in the future to prevent this sort of nightmare from happening again.