Fighting sippy cup

Hello, my baby is 15 months. We transitioned her to a sippy cup from a bottle when she was about 11 months. She has never complained until a couple days ago. She refuses to take it. She will cry and cry (because she is sleepy and drinks milk for going to sleep) until I decided to test it I added milk to a bottle and gave it to her and she took it and went to sleep like she normally would. I thought she was just refusing to sleep but now I’m thinking she’s fighting the sippy cup. Any one gone through this or have any tips?