She may also be teething and it hurts to hold the cup in the mouth to drink. Take any no spill insert out so the milk flows freely and more or less pour small amounts into her mouth.

As weird as it sounds use the medicine syringe. She is used to breastfeeding which means once she latches on the milk squirts onto the roof of her mouth and she swallows it. Fill the syringe and squirt. Just keep repeating till she seems to want to hold the syringe in her mouth and suck and then try a cup without the no spill insert so it flows freely - just watch how freely you let it flow so she doesn't gag.

Make sure you are warming the breastmilk and quite warm as that is the temperature she is used to from breastfeeding as in slightly warmer than body temperature.