It definitely sounds like you are describing is colitis or inflammation of the large bowel. The signs can be very intermittent and can vary in severity, from normal stools to mushy to very soft, all even within one day at times. Seeing mucus or blood is not unusual as well in some cases. There can be a sense of urgency to move their bowels and many of the pets will strain when having a BM, which to the owner can almost look like they are constipated. Many cats with colitis have GI discomfort. Cats are very sensitive and if she associates her litter pan with GI pain, she will most likely not want to go back to the litter pan for stools and she will find other spots in the house.
Colitis can have many causes including intestinal parasites, dietary indiscretion, food allergies or intolerances, metabolic disturbances from internal organ malfunctions, viruses, toxins, cancer or even stress. In cats, colitis can also be part of the disease process called inflammatory bowel disease (IBD) in which we often never identify the cause of the inflammation. Your veterinarian will want to know the history of her signs and will need to perform a physical exam to help rule out the causes mentioned. They may perhaps suggest a change in diet or medications. Make sure they also perform a rectal exam to evaluate her anal glands. A stool sample analysis is a very important part of getting to the bottom of this as intestinal parasites are one of the most common causes.
Until you can get in contact with your vet, I would fast her to give her GI system a rest. Since he is an adult, a 12-24 hr fast will not hurt her and will let her gut calm down. Continue to offer small amounts of water at a time. When you do begin feeding her again, you will want to offer frequent, small meals of something very digestible for her. These would include items like boiled hamburger or chicken. No sauces or gravies, but a little non greasy broth would be OK.
When they have been going on chronically, these GI disturbances can be frustrating to diagnose and treat and sometimes we don't even find the cause, but concentrate on controlling the symptoms so our pets are comfortable. In her case, since it appears to be an acute flare up, there is no reason to think that it won't resolve completely. Make sure to get in touch with your vet if she is not improving so they can get her treated appropriately and verify that there is nothing else going on to be causing these signs.