I had sex with my ex girlfriend on march 22, I used protection but it broke. She told me that she is pregnant ?

I had sex with my ex girlfriend on march 22, I used protection but it broke. She told me that she is pregnant and a blood test showed her at thirteen weeks and gave her a due date of December 19 just a week ago. On Friday she went to the Dr and they told her she's due December 6 and that she's 15 and a half weeks. I asked her if she's still sure its mine and she said yes because the blood test are more accurate that sonograms because sonograms are based on measurements and you never know how babies will grow. She also said a couple weeks ago amniotic fluid was gushing out but then told me it was not enough to worry about. I'm very confused about the whole thing.