Someone please correct me if my memory is wrong here, but my recollection is that after Billie was raped, she left town for a couple of weeks to get away from things, & then slept with VJ virtually straight after she got back. Then only a day or so later, she was feeling sick (after teaching a gym class?) & the lady doctor (who was with her after taking Billie's gym class?) took her to the hospital, where after taking tests, told (a shocked) Billie that she was pregnant & would have conceived a couple of weeks ago. This supposedly confirmed, unofficially, that VJ couldn't be father? & then after she discussed with Phoebe what had happened, prompted Billie to eventually sneak in & change the conception date on her hospital records to the date she slept with VJ. At this stage, an actual/official paternity test hasn't been done though.