Given that all of her blood work was within normal limits, I would have to agree that her tummy upset could very well have been from eating the roast beef. It most likely just didn't agree with her. I would try to find a food that she likes and does well on and stick with it to get her tummy issues settling. This may be more difficult than it sounds with so many different types of pet foods out there and all of these recalls to worry about too.
I would try to give her a little bit of pure pumpkin about a tsp with her meal which I know always helps my girls tummies settle down. They love it too!
Bless you for taking in this little girl that you fostered. I'm sure once she's been in your home she will start to feel more relaxed and her anxieties will start to improve. It's bound to take time given what she's been through.
