You have to assume BB no matter what the girl says or does. For all we know, she could well be scrupulously clean and has never had sex with anyone without a condom..... apart from the one time a condom split. It happens and then the girl has had unprotected sex.
I've had a condom split on me before. I stopped punting until until I'd been through all the tests and allowed them enough time to be effective. I noticed that the girl didn't stop working though. So no matter what they say about always using a condom, you have to remember one thing....."shit happens".