Hello, I'd like to help you with your question.
It sounds like your supervisor has a personality disorder. What makes me suggest that is her changes in personality and attitude towards you when it suits her.
Someone who has a personality disorder thinks that the way they are acting is right and the other person is wrong, no matter the circumstances or personal relationship. They do not feel your concerns are valid, no matter if they are or not. They also like to manipulate, or play games with the other person's feelings. And if this person holds a position of authority over you, this can make working for them very difficult. Their ability to adapt their persona to whatever situation they are dealing with also makes it hard. They can seem competent and steadfast with their own supervisor, then turn on you when they work with you.
One thing you can do is learn more about personality issues/disorders, especially in the workplace. Although most people with a personality issue have little insight into what they are doing and therefore have a hard time changing themselves, you can cope better by knowing why they act that way and how to respond in ways that are best for protecting yourself. Here are some resources to help you:
http://www.ehow.com/how_2066493_help-someone-borderline-personality-disorder.html- this is also good for other personality disorders
The Wizard of Oz and Other Narcissists: Coping with the One-Way Relationship in Work, Love, and Family by Eleanor D. Payson
Toxic Coworkers: How to Deal with Dysfunctional People on the Job by Alan A. Cavaiola
Dealing with People You Can't Stand: How to Bring Out the Best in People at Their Worst by Dr. Rick Brinkman
You may also want to start keeping track of how you are being treated. Keep a notebook with you in your pocketbook (leaving it in your desk may not be good to do in case someone sees it) and jot down the date, time and brief description of the situation. If you can get enough, you can try either talking to Human Resources or if you don't feel safe there, the EEOC. They can instruct you on how to proceed with filing a complaint against your employer.
If you can, get your co workers to witness what this supervisor does with you. Try not to be alone with her if at all possible. Leave your office door open or have a co worker listen nearby. And when you can, try to avoid her, like you have been doing. Or even ask to be supervised by someone else. Any way you can stay out of her path is best.
Also, consider getting legal representation for yourself eventually, especially if this escalates. It will be good to have someone in your corner who has known all along about what is doing on. Neighborhood legal services may be able to help if you do not already have an attorney.
And don't forget to try ways to relieve your stress from this. Work problems can escalate your stress and you may not recognize it. Take time for yourself to relax, talk out how you feel, and treat yourself. Get a massage, take a long bath, spend time on your hobbies, and get as much positive reinforcement as you can from your friends and family. Work can take a toll on your self esteem and finding ways to boost it can help you approach this problem with a better frame of mind.
I hope this has helped you,