Story changing

There’s a story in the local paper about a Belfast woman who confessed to murdering her husband has changed her plea in the case from “not guilty by reason of mental defect” to simply “not guilty.”  The story then talks about how the woman had confessed to her neighbor and to the police that she had killed her husband.  So I don’t understand why she would think for a minute that she could start out with a confession and then change the plea to not guilty.  Doesn’t seem to make any sense at all?  Can you explain it to me?