You have made a major change in the way that she gets milk. It sometimes takes time for some babies to adjust. And if she's also teething she may not have much of an appetite. I wouldn't be too concerned.
It is true that babies will eat when they're hungry. Three days with "barely eating anything" is such a short period. You will find that she will have more days like this as she gets older.
My 6 yo grandson still sometimes goes days at a time with apparently not eating much and then chows down "like crazy" several days later.
It is good that you have a doctors appointment so that you can be reassured. In the meantime try not to fret.
She may be having difficulty accepting an alternate way of nursing in part because of your tension and anxiety. Babies are very good at picking up on that tension and becoming anxious themselves.