Hello, I would be happy to help you with your question.
Assuming that you have always used backup protection, and assuming that you did not ejaculate when you did not use condoms, then it sounds like you could not have fathered a child with her. That being said, there is always a chance of failure of birth control, regardless of the method. Consider offering to take her in to a clinic to confirm the pregnancy
. It is appropriate for you to ask for confirmation of some kind. Do not resort to deceptive tactics. She owes you a straight answer.
I hope that this helps. Good luck and thank you for visiting.