Yes!
It can happen. In fact it happened to a friend of mine. Understand that sperm can live up to 7 days, sooo, if you had sex at the end of her menstration cycle, your boys could still be swimming for a while. Sperm will generally fertilize the egg while it's travelling down the fallopian tube, so if she has an early ovulation cycle.....bang.
The risks of all those factors coming together though are VERY slim. Keep in mind that there are generally only 2 days per month that a woman can get pregnant..... So you're probably pretty safe. I hope that helps..
Steph
P.S. A woman will *usually* ovulate 11-13 days after the start of menstration....