Regression---Help?

My 8 month old female ShihTzu was doing really well with her potty training; telling me when she had to go out, rarely having any accidents. Then she started to regress; didn't tell me, would go behind a chair in the family room, or in the sitting room and sometimes poop, sometimes pee. Only in one of two places. I have scolded her, and taken her outside immediately. She seems to understand, but it has happened several times now, if I don't anticipate her. Now I don't trust her. Any suggesions? Do I have to start alll over from the beginning and keep her confined in snmall areas? How do I prevent her using those two areas as her "potty spots"?: