If you haven’t done so already, I would suggest telling her you don’t like being slapped. Respecting each other’s boundaries is a pretty fundamental part of maintaining a healthy relationship.
You have a good point there. If you feel threatened by the slapping, and/or she won’t stop after you’ve asked her to, then it does become more of a domestic violence issue. Find BC and Boston help/resources here: bit.ly/BCPartnerAbuse.