I would have said stroke, but I don't see labored breathing, etc. Strokes in rats are almost always associated with labored breathing and lethargy (days before they lose motor function). I just lost a very special rat to a stroke and he was only 4 months, so it could be a stroke. I'm leaning more toward neurological or inner ear infection though.

Is it possible her head was crushed or she had some other form of blunt trauma? I know some rat racks are difficult to close and accidental injuries can be caused without knowledge from the keepers.

On a personal note, if you notice she is suffering and still has the same symptoms tomorrow, I would euthanize her.

Anyway, good luck.