Can A Person Impregnate A Woman Post Vasectomy?

I had an affair, girlfriend was trying to get me to leave my wife. Now girlfriend claims to be pregnant. I had a vasectomy 9 years ago, so just to be sure, I got tested. Just got sperm count analysis with none seen result. Is it possible that I got her pregnant, or has girlfriend been cheating and just trying to sabotage my attempt to fix things with my wife.