I agree with Flea, I reckon it is because she didn't like the idea of doing it in a bucket. My DS was the same with TT in a way because he just couldn't go on someone else's toilet for #2's and he would just hold it in, which gave him belly aches. Give her loads of fruit and water to soften it a bit (cause no doubt it will be a big one) and sit her on there and read a book or sing songs just to keep her on there long enough to relax and do one. I think she will be right once she gets the first one over and done with.