My daughter went through this. She was potty trained before she was 1 yr old. I was divorced and lived with my parents till I married again, so while I worked my mom potty trained her. After I was married about a month, my daughter started peeing in her panties. Drove me up the wall because I knew she knew better. Finally someone told me that a child regresses back in their learning sometimes when there is a change in their life or something dramatic happens. Mine was marrying and changing enviroments and her having to adjust. I guess to a little child that's a big thing when you change from one family to another. So if there has been any changes in her life than what she is used too maybe that could be it. After awhile she started telling me when she had to go pee but till she did it was so frustrating. I just had to let her do it in her own time. Good luck.