Inappropriate soiling is often an early symptom of a health problem that could be anything from intestinal infection to urinary disease.
She should be seen by her vet. Since this is a rescue, there should be options for subsidizing her care through your vet or any number of rescue orgs (just ask the vet or call the org that placed the cat with you).
In any case - this is a symptom that always needs medical evaluation first - and if she comes through with all negative reports, pursuing a behavioral cause would be next.
Please check back and let me know ok?
(and thank you for rescuing this girl! Good for you, better for her!)
Many have forgotten this truth, but you must not forget it.
You remain responsible, forever, for what you have tamed.