It could well be an inner ear infection. If so you can treat with antibiotics but must do so immediately or she may be left with permanent problems. If you decide to cull her instead then you will never know the cause and whether to breed from that line is subject to the possible problems you outline above. Whereabouts are you? Perhaps you could get some new mice to breed from, since it sounds like you are having inbreeding depression with your current lines anyway from what you have said.