Every time I have one of our old dogs start doing something out of the ordinary, I pull blood on them and everytime we have found something wrong. Most of the time it's been a tick born diseas and one had swine brucelossis. By catching it quick, they got right over it. This is where it pays to know your dog.
Try this. Like bigO said, you know that dog, if it's out of the ordinary, start looking her over real good.