We recently went through this with our 6 year old. He loves school, but started giving me a hard time about going. I talked with his teacher and she said everything was fine in class and when he was there he had a great time. We talked with our son and he just kept saying when he is in school, he feels like he is sick. But he would never go to the school nurse. It took about 3 weeks of this (every day!). Finally, after trying to talk to him and understand what is going on, we figured out that he sits with an older boy on the bus in the morning who is bullying him and others. I spoke with the bus driver, he rearranged some seats, spoke with the bully, and my son has been more than happy to go to school ever since!
So look at all aspects and don't give up! By helping her figure out what's up and how to fix it, you'll help her start to enjoy school.
Good luck!!