C.S.
My experience is that you'd know for sure. Does she have it? How old is she? Sometimes the way their head is tilted w/the suction hurts more on 1 side than the other while teething... or could be too full on 1 side, maybe hand express to soften it a bit??
If you think it is thrush, otc monistat cream works well. However when I had thrush my son still wanted to nurse...