It sounds like she's pissed that she now has to share an office with you. If her attitude is bothering you, see if you can move to another office where your co-worker might be a bit more friendly. It also could be that she is shy, doesn't like talking, or she isn't used to someone being in there with her all the time. Take a page from her book and just say "Morning" or "Bye". Anyway, just grin and bear it. Who knows, maybe you'll get promoted to an office all by yourself.