I got a vasectomy! Can i be the dad?

I got a vasectomy in April of 2010. and went for a check up after and they said the there was no semen found. About five weeks ago had she is not on any birth control and did not us a condom and shes late what are the chances if shes pregnant it is by me ( she slept with a few guy with in a few weeks before me) . i thought there is only a 1% chance. i dont want any bull just facts thanks