Welcome to our forum. This problem is common among multi-cat households. How many litter boxes do you have? You need FOUR and they need to be spread out the home -- not all in one spot. When medical issues have been ruled out, it is usually stress related. Many times it is a territorial thing. Could there be outdoor cats coming around your property that your kitty can smell or even see? The territorial problem is more with males than females. However ... there is a problem somewhere! It sounds as if all your kitties get along, but is there ever any aggression between them? You might check out Jackson Galaxy's Spirit Essences
http://www.spiritessences.com and see if there is a remedy that might help you out.