Unless she provides useful services to you as a co-worker, just leave her alone. Seriously...just ignore her. Not in an unfriendly way. Just leave her be. Don't stew about it...she'll either get out of her mood or she won't or she'll confront you with whatever is bothering her. This happens to me and I spend way too much energy trying to figure this shit out and in the end, it hurts my productivity at work. If she is too valuable to your work, then suck it up and just tell her how you feel, direct and to the point, i.e. "why have you changed how you treat me lately?" If she refuses to acknowledge there has been a change, drop it and do your best to work around her bullshit.