S.B.
Find a counselor for your daughter that specializes in children of divorce.
Little kids are not equipped to handle their emotions and the changes that are occurring, let alone be able to understand their parents moods, sadness, etc.
Do NOT make any assumptions other than your daughter needs someone to help her with her thoughts and feelings and how to navigate the choppy waters of being a child in her shoes. The counselor can also help you with answering questions your daughter poses, and how to handle certain situations as they arise.
The counselor can get to the bottom of where anxiety is coming from and how to deal with it.
Start checking around on Monday. Seriously. Your daughter has the rest of her life to cope with the fact that her parents are no longer going to be married.
Best wishes.