Changing the food is disorienting, and as Paige mentions can well cause stomach upset. Do not be surprised if her poops are quite loose now she has suddenly changed her diet like this.
I am sure she has chosen under the bed as being more protected. When you come in to feed her have you been attempting to talk to her and interact with her? I think giving her some space makes more sense at this point, she is obviously quite thrown by all this change. Be aware that staring at her is not reassuring but a sign of aggression for cats, you want to only give quick glances or be sure to blink slowly to reassure her.
I would schedule her spay asap. The sooner you are not dealing with hormones the better, cats tend to settle better after a spay also, so it will help her become the pet you want.
Given she was a breeding cat, how did your friend keep her? Some have their breeding cats as pets in their house and others keep them in cages and so what is your cat used to? This can be useful knowledge and might tell us how quickly you might expect her to adapt. If she has grown up in a cattery situation not a home pet situation then this is a much bigger adjustment for her.
I agree with Paige that having some ambient noise such as radio or tv can really help. There is a "too quiet" for a nervous cat..while some background noise can be soothing. Give her a couple days more of little interaction then you might just go sit in the room quietly for an hour or two at a time, read a book or watch tv but continue to ignore her. Sleep in the room possibly, sometimes they find the middle of the night a good time to come out and sniff you over as when you are asleep you are less of a threat. It might not be a peaceful night for you if she wanders around a lot but it might help her adjust to you.