Try not to make a big issue of it and try again in a few weeks. Even as far as not talking about it if she does one in her nappy or knickers. Clean her up and not try to show any emotion (upset/dissapointment etc).
I know that's hard (Angus had major TT regression when Caleb was born and pooed in his pants every day) but if you act like it is no big deal then hopefully she will get over any issue she has with it herself IYKWIM.