My cat sways her head and falls down?

When I got my cat from the humane society, she had been abused and suffered from brain damage. Every so often if she's walking, sitting, eating, whatever, she'll sway her head sharply to the side, and she sometimes falls down. It doesn't seem to bother her that much, and she checks out at the vet as having no major problems. I'm just curious to know what this might be. It makes me think there's some inner ear abnormality or something wrong with her balance.