I feel for you! Try not to think of it so much as leaving you, maybe she just needs to live with her dad for a bit to reinforce her feelings on that side. As much as it hurts, it may have nothing to do a choice between the two of you. She may feel more secure of her relationship with you but not so much with dad. Maybe she feels some time with him might give her the feeling he loves her. I think every child not living with one parent or another, will question their love from the absent one. Moving in with him for a while may ease those fears and also reinforce her need to be back with you.
Of course then again, it might be that she is rebelling against your authority. Every daughter goes through that I think. Unfortunately, in a divorce, it gives them another "out" to go to instead of working through it at home. I hope you are able to talk seriously with her dad and lay down some ground rules. You can't let her think she can just bounce back and forth whenever she might be displeased with a parenting decision. I hope the dad will work with you on this.
If it helps, I think every daughter has some phase where they do not get along with mom. I must have given my mom fits as a teen, but I love her endlessly! I really appreciated how she was strict with me after all was said and done. But at the time, I would never thought like that. Life was just not fair!
I could always pout with the best of them. Best of luck with working through this with your daughter.