Have you brought in any new cats?
Does she go outside at all?
Has she been chewing any houseplants?
Have you changed foods recently?
What breed of cat is this?
It sounds like there are a couple of possibilities. One is that she has picked up an intestinal parasite at some point when she was outside. Your vet can check a fresh stool sample for microscopic worm eggs or cycts of protozoan infections such as coccidia, giardia or other bacteria.
Another possibility is inflammatory bowel which you can read abuot here
http://www.marvistavet.com/html/body_inflammatory_bowel_disease.html
A cat can develop an intolerance to a food they always eat which can trigger this problem.
If nothing is found in the stool sample I'd plan a vet visit for your cat. She may need fluids, injection of a stomach soothing med etc.
Hope this helps you!
Pet Health Care, Rescue,Train,Breed
30+yrs pet vet care & nursing, rescue, behavior & training, responsible breeding, small animal care