Who sexed the female? everyone makes mistakes. Even some of the larger breeders on here have had females end up being males that they themselves could say that they were 100% female. It never hurts to double, triple, even quadruple check, because until you see penes, there is always a possibility that a 'female' is actually a male with some strong muscles holding it all in....
PS - Ball Pythons aren't Pregnant they are Gravid.