We had that happen once with a foster dog. He was the smallest (ours were way bigger) but he would snap at them at the slightest thing. It seems like it was more of a defense thing and luckily he never snapped at us. Was always very loving with people.
He went on to a different foster home and got along great with their dogs. Either just a different chemistry or maybe he just needed longer to settle in.
Did they give you any history on Bella in her foster home (other dogs, etc.). She might just be freaked out right now and get better as she feels more secure.
I'd still call and let the rescue place know though. Maybe they can help.

Allan! With your love of everything manly I would have never pictured you with a purse dog under your arm. Pink sparkly collars too?