The sudden change is a red flag for several things, most of which you have identified.
Her high scholastic achievement is a good sign: when you get her into counseling, and you will, she has the brain power to make some use of it. So often we think that counseling helps everyone, but it doesn't..... You have to have enough IQ points to reason, discuss, develop alternatives, implement them, and evaluate them. (Not unlike nursing process....)
Bipolar disorder certainly is a possibility, but adolescents who are depressed also act this way. When they are depressed, they may lash out at you for one thing, but it's something they can't bring themselves to put into words that has them upset--or there isn't anything specific, they are just depressed.
You are right to be concerned--I want you to know I think you are an exemplary parent. How you are keeping such a cool head is beyond me--you are better at this than I was when mine (then 13, now 21 and quite sane--so far!) was acting out. It sounds like you are doing all the right things.
Does your daughter have anyone that she has been able to talk to in the past--a teacher, counselor, Sunday School teacher, pastor, neighbor, friend's parent--that she might be willing to talk with if she were assured that it would be confidential? It's just a thought--sometimes that breaks the isolation.
It really sounds like you are on the right path. Don't forget to take care of yourself too. Get plenty of hugs from the DH. These kinds of things can strengthen a marriage if you can draw strength from each other, and that helps the kiddos too.
Take care--stay in touch.