If all of her poop is gooey and mushy, with no normal round fecal balls being produced, this is true diarrhea and is considered an emergency. It's caused by pathogenic bacteria, usually the clostridium bacteria, which causes enterotoxemia. This is a very serious illness and very often is fatal. To have any chance of saving a rabbit with this, immediate treatment and the correct meds are absolutely vital. If this is what she has, she needs to be immediately put on cholestyramine to absorb the toxins produced by the bacteria, as this is what proves fatal to the rabbit. Then also the antibiotic metronidazole or possibly other antibiotics(rabbit safe ones, as not all are). She'll also probably need sub q or IV fluids, syringe feeding if she's not eating well on her own, and may need some additional medications.
www.mspca.org
Medirabbit: bacterial and mucoid enteritis, enterotoxemia
Medirabbit: antibiotics safe for use in rabbits
If she is still producing normal round fecal balls, then this isn't usually an emergency, but can be serious if not corrected. When there is some mushy poop, but also normal round fecal balls, the mushy poop is usually mushy cecotropes that aren't properly formed. This is called cecal dysbiosis, or intermittent soft cecotropes. Most often the cause is a problem with the rabbits diet. Usually too many sugars/carbohydrates, and not enough fiber from hay. Though sometimes it can be caused by a sensitivity to a particular veggie or green fed, or veggies that have spoiled. This can usually be corrected by a diet low in sugars/carbs and high in free fed good quality(no mold) grass hay. Occasionally ISC isn't due to diet but can be from other problems like liver disease, obesity, dental problems. So if it's not a diet problem, your rabbit will need a thorough exam, dental exam, and may need blood tests and xrays to determine the cause.
rabbit.org
