Most Helpful Girl
With the situation you've described it usually means one of two things: extreme guilt, or complete denial. She either did something that she feels really bad about, or you did something so shocking that she doesn't want to deal with it/you at all. She may have cheated with this new guy, and rather than face you, she cut it off completely the second after it happened.
Most Helpful Guy
if i was u, i would consider it over since she's with another guy :-)