Hello and welcome to Just Answer. I am a licensed veterinarian and would be happy to answer your questions.
What it 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. There is 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 pets with colitis have GI discomfort and some will even vomit. It is also common for the abnormal stool to be very odorous.
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. Since it sounds like parasites have been ruled out it may be good to pursue a different cause. Although, some parasites are harder to identify and special tests are needed. It is also easier to miss some of the parasites if the stool was very liquidy when it was checked. Bloodwork is often normal in these cases and sometimes response to medications are used to confirm the possible cause.
I would talk to your veterinarian about ruling out a food allergy with a limited ingredient dietary trial. She may also benefit from treatment with the medication called metronidazole and some probiotics added to her food. Some cases that are more chronic or severe may end up needing treatment with steroids, but there are other things that can be tried first.
I would definitely work with your vet to see if you can get this resolved. 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. 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. Please let me know if you have any further questions.