This sounds like a vestibular issue. http://www.vetinfo.com/dencyclopedia/devestib.html If I were you, I would take her to the vet ASAP. In the mean time: -Does she respond to sounds the same? Maybe you could rule out an inner ear infection to see if her auditory system is still up to par. Unless you're seeing other symptoms, I'm going to say that a cerebellar tumor is unlikely. (I work with human neuroscience, so I may not be the best source of information) The auditory and vestibular higher centers are amazingly plastic. Your dog's brain might adapt to the the head tilt and fix it within a few days.