My girlfriend is pregnant and she said its mine

I'm afraid the answer is both yes and no - she can roughly know the date when she gotten pregnant because that can happen only when she is ovulating (ovulation lasts for 2-3 days just in the middle between her two periods). Obviously, you two had to have sex exactly during that time. But, on the other hand, how does she know she is pregnant so soon (like 13 days after the sex)? Home pregnancy test become accurate at least two weeks after unprotected sex. Did she see a doctor?