Mine went from nursing, to formula when I got sick, to goat's milk, to lactaid then finally cow's milk when he was two. At 14 he has switched to 1% and 2% milk, and occasionally goes on chocolate Silk binges.
Maybe try another milk - there is goat, soy, almond, rice, and I know more. She may not like the taste of the cow's milk anymore - I know I can't stand cow's milk now after 30+ years of being a milk drinker. :) It was a novel new taste at first, I am sure, but she may just have realized she doesn't like it.
Don't go the added flavours route - then she will not drink anything but flavoured milks - I had a friend who did that and her daughter at 11 years old still has trouble drinking plain milk.
And it is probably a wanting Mommy thing. I think nursing is more than just food for our babies - it is warmth and closeness, our scent, and our heart beat. Try holding her on your lap when you give her the cup to drink from - associating being close to Mommy with drinking the milk from the cup, not from Mommy.
Most of all, just be patient. She will adjust no matter how strong willed she is.
Good Luck and God Bless