Did the condom fail? Did you take it off or put it in without one at any point? If she does get around, and you were actually safe, yeah it could be yours, but I would imagine better chance then not it isn't yours. But that is based on the honesty of your post.
With that said, I never cared if it ruined the moment, even if I was drunk I always checked condoms after the fact. To the sink filled it up and made sure it wasn't leaking.
But... have you tried acting like adults and simply talking to her? I don't mean that rudely but I've known both guys and girls who get really freaking stupid when it comes to the chances of having a kid.