We had one that did this same thing at 14, the vet said stroke or tumor and put her on steriods. She did recover so he determined hers was a stroke. A tumor would have continued to get worse. The reason the tumor can be so sudden is as it grows it gets to a size where everything is pushed as far as it can go and it is now pressed up against something and causing problems. Our girl was circling to the left, staggering, same thing. She slowly recovered.