Fiona
It could be that she is still setting in and the impact of the change of food. Or could it be that she has eaten something that she shouldnt??
Good cures for the runs are a desert spoon of carob powder, or slippery elm. My Labs are always eating things that they shouldnt and unless it is really bad I just add the carob to the next meal and all is fine. I save the slippery elm for really bad bouts!
2 things to watch out for 1. If it has a slimy mucus 2. If it has blood in it. Both of these should be a trip to the vet.
Above all it is importain to make suer that she remains hydrated, if you lift the skin on the back of her neck it should spring straight back, if not she is dehydrated and again needs to go to the vet.