Silly question but have you asked her why? I'm have heard of kids being afraid of the plopping sound.
Maybe buy a potty & decorate it together & make it her special place until she gets used to having bowel movements again.
I know my 4 year old nephew refuses to do one at daycare & will hold it until he gets home.
DH 37 + me 33 = DS1 March 2010 & DS2 July 2013