What's your companies context - amdram, educational, professional - non union, professional - union?
Are you the only one experiencing these issues? Are the rest of the cast and crew feeling it too?
You said that she feels like everyone is questioning her - does she seem to react poorly to anyone in particular? is there someone she seems to trust?
The best method is to have a discussion away from the rest of the cast and crew if you feel you have the best rapport. If you believe the relationship has devolved too far for a frank conversation to have any effect, then maybe you need to go up the chain.
The one thing you _NEED_ to get across - either yourself, someone she trusts or the production manager - is that you are there to support HER. You can only support her if she works with you, allows you to do your job and etc. Also emphasis on the fact that whilst during the rehearsal and design phases changes are certainly possible, in order to get the production running to track it is important to try and maintain a given course in design. If the design isn't working, then there is room to change. But it should be done in consultation with the other departments to ensure that the changes made are possible.