I would call the RSPCA you got her from just to let them know what is happening. Watch her stools for any blood. If there is blood then she needs to go to a vet straight away.

It may just be that she has a sensitive tummy to certain foods. I have a dog like that that cant have our left overs or egg as he has a very sensitive stomach from when he was sick as a puppy.

After he got really sick with parvovirus when we first got him, they recommended offering him plain chicken in very small pieces when we took him home.

Hope sshe is okay

Give her lots of cuddles so that she feels loved and safe in her new home