Is she still producing normal round fecal balls for the most part, though maybe smaller than they normally should be, or is it all mostly mushy poop? How long has she been with you and how long has the mushy poop been going on?
What is her exact diet(type and brand of pellets, hay, treats, supplements, etc) and how much of it is she eating per day(tsp, tbsp, cup, etc)? If she is a new rabbit for you, are the pellets and all her food, the exact same type and brand as she was having in her previous home? Do you know for sure if she is drinking water ok? If she only has a drip bottle, you will need to check that it isn't blocked up and stopping her from getting water. A rabbit not able to drink, also won't eat. If she only has a bottle, also offer a water dish.
What is her approximate weight, is she smaller than her siblings and how much do they weigh(if you know this info)? When feeling along her back and hindquarters, does she feel thin and boney at all? Does she seem to have a pot belly? When is the vet appointment?