I would say if it was when her period was then your chances are very low. When she has her period she's not fertile. But I never say never. I wouldn't worry about it.
If she was pregnant she would already show positive on a test. Also, it is extremempy unlikely to become pregnant while on your period.
for how fertile she is on her period would depend on how many days into her period she was and how long her cycle is... without knowing a due date or if she already knows who the father is its hard to say for sure...