I don't have as much experience with cat rescue, but a favorite behaviorist, Patricia McConnell, noted that one of the most important things to have while integrating a rescue into the family is "patience, patience, patience." She was talking about dogs, but I would think it would go doubly for cats. Looking at it from your kitty's point of view, this is her third home (hoarder, foster/shelter, yours) in a short period, and she doesn't know how things work yet (who are these people, strange cat smell, where is it safe). Kitty may be somewhat shy and skittish by nature (we have an 8 year-old boy who has never been without shelter a day in his life, but he still startles at a sudden noise) but has come around pretty quickly to you. Our kitty hid in the cupboard for two days when he first arrived.
You'll probably continue to have times when she regresses, but take heart that she'll come around. She'll probably play when she feels relaxed, but even if she doesn't, if she takes treats from you and wants petting, that's even better. Be very slow in introducing her to your new cat (Jackson Galaxy's protocol is good). If it takes weeks, or even several months, that's okay.
Thank you for giving her a loving home.#AdoptingaPet