It could just be from the food switch, I wouldn't switch her again (although, you could pick up a very small bag of what she was on before and mix it in with the Acana just to help make the trasition easier), that could make it worse.
Try adding some canned pumpkin (UNspiced) to her food and see if that helps firm it up.
If it doesn't get better in a few days, talk to your vet. You should also have the vet do a stool check for parasites asap if you haven't already
