Have her checked by a vet tomorrow. She could have an ear infection, vestibular syndrome, or any number of other problems.

What kind of dog is this? There's a fatal disease called wobblers disease, my friend's doberman died very suddenly of it. It affects larger breeds. Please get to the vet asap! According to wikipedia, it can be treated surgically now. Hopefully this is an ear infection, ear mites, or something much less serious as annz suggested.

Good luck, and please let us know what you find out.

Here is a link that might be useful: wobblers overview

Wobblers isn't always fatal, but it certainly can lead to serious problems getting around well. I have treated about a dozen dogs (nearly all dobes) with Wobblers and yet had one die from it... though some eventually had a lot of difficulty getting around. Rarely causes a head shake, either.. the wobbly part is the rear end (causes some neck pinching but there are various degrees of impingement from very mild to severe- most cases tend to be more on the mild side).

Your dog does sound like it might have something in its ear... foxtail (grass awn), or infection most common things to find in there. Your vet should be able to help out if that is indeed the problem.