Rise above it. I'm not being trite when I say that either. Visualize yourself looking down on the incident and see it for what it is. In the grand scheme of things, it's insignificant. To be sure, it can be difficult to work w/an unpleasant co-worker, so I'm not trivializing your feelings in the matter but telling you that you have a choice here to either let it affect you (and probably give her satisfaction) or just to get on w/your day and let her see that she isn't having the effect on you that she probably thinks she is.