This is very interesting because my son had thrush at 15 months for about a week before I took him to the doctor (I thought it was a canker sore). I was nursing him all this time and I never got thrush anywhere. The day the doctor diagnosed it was thrush, the night before was our last nursing session. I never ended up getting thrush. My sons cleared up in about two weeks. I constantly sterilized everything, even after his mouth cleared up for probably about a month after.
Maybe you could check with the pediatrician to see if it's okay for your daughter to take the nystatin as a preventative? I remember my doctor saying it's not absorbed into the body so it's safe. Or, you could start her on probiotics to balance out her bodies' natural flora.
Good luck.