I had her back to the vet today because this was dragging on, and now have probiotics and antibiotics to try to get her tum back in shape. Vet thought she'd eaten something that disagreed with her, and I couldn't work out what that could be, as I've tried to stick more or less to what she was eating with her previous owner (Butcher's tinned food and cooked lamb's liver).
I've just realised, however, that on Wednesday evening, I gave her a very small dried pig's ear, which she ate with enormous enthusiasm. I bet that's what it was. DOH! Major mistake!
Anyway, fingers crossed that the various treatments will sort her out. And note to self: even if foster dog does have rather stained teeth, pig's ear is a high-risk way to try to clean them! Stick to tripe.