"Friend" Troubles and in need of help!!!?

This girl (lets call her Giselle) was my friend for all of freshman year and 1/4 of sopre year, until she suddenly screamed at me for no reason at all one day. A few weeks after that, she acted like nothing had happened and we were best friends again. Truth is, I was sick of her. So now, in the beginning of junior year, Giselle walks into my homeroom and starts yelling and swearing at me, again for no reason. I am extremely tired of this, but thing is I am extremely shy and unertive. Any advice on what to do to get this girl off my back? Thanks. :) Suggestions are appreciated.