More than likely she's very stressed. She doesn't understand why her mother and siblings are not there.
Being so little, Mom probably never taught her proper potty etiquette. You will have to do this for her, and unfortunately I really don't know what to tell you about it.
I'm sorry. I did a search and didn't find anything that would help. Perhaps it would be best to post your question in the Behavior Forum where our experts are more likely to see it.
Of course, Hissy is one of our experts so I would definitely start out with her advice.
She is a real cutie, and for some reason Portia seems to fit her to me.
More than likely she is a mixed breed, especially seeing as she was dumped. Some irresponsible pet owner didn't want their cat to have kittens but didn't get Mom spayed.