She could be fertile three days after her period, but most women ovulate mid-cycle, in which case it would be more reasonable to assume that the other man is the father and NOT you. Depending on the length of her cycle I can almost say with certainty the other man is the father because a couple weeks into the cycle would put her at just the right time.

Since you wore a condom (tho not 100 percent effective at preventing pregnancy) then that means that you have an even lower possibility of being the father.

Don't let her try to convince you that you are.....I mean if you wanted to be the dad of the kid that is ok, but it's best to get a paternity test for many reasons other than this. I don't know why she wants you to be the father.....but don't let her push you around about it until you have proof of a DNA test, which is pretty easy and simple to get after the baby is born.