If you have uncomfortable concerns about your flatmate, then you probably really shouldn't be moving in with her.
Saying that, if you live with someone as a flatmate, then you don't have the right to dictate who she brings into the home. However, out of common courtesy, she should really inform you if she plans to bring a friend back and vice versa.
It's a difficult situation, which is why I never flat shared. Perhaps you could consider renting a small studio or renting a room in a family home or even at the YWCA.
If you still plan to move in with her, then you could have a lock on your door, so you feel safer.