I'm so sorry to hear of this abuse. I also suggest you not confront this abuser, but go and tell someone in the position to protect you.
If you feel the person to whom you would report this behavior (her employer? a family member?) won't believe you, see if you can borrow a small tape recorder and have it in your pocket when she is around. Some recorders can even be set to turn on when a voice speaks. That way you can have a way to prove she is doing what you say she is doing.
Ange, I also wonder if there is someone she answers to, and what this situation is that you're in. Are you in a group foster care home and she comes in for her shift?
Yes, this is abusive behavior. But I think one lesson you could learn from this is, if someone is abusive to you it's not in your best interest to back talk them if they have power over you. Telling her that her words were over the top and raising her voice doesn't make you listen any better is not going to result in a positive outcome for you. If you were dealing with someone who was rational and who was trying to do her best, feedback is sometimes helpful. If you're dealing with a bully and she has power over you, don't take her on yourself. You will lose.
Best wishes. I agree with Annie, best to go to her employer, whatever your situation is.
She is your caretaker? She shouldn't behave this way. Is there anyone to whom she answers, that you could talk to about this?