A change in mentation (mental status) as you've described at 10 years of age might represent cognitive dysfunction (senility) and both intracranial (within the skull) and extracranial (outside the skull) encephalopathies (brain disorders). Extracranial encephalopathies would appear to be unlikely because she's otherwise normal. These usually involve the liver (hepatoencephalopathy/a poorly functioning liver is intoxicating the brain) or kidneys (poorly functioning kidneys result in uremic toxins circulating in the bloodstream which then can intoxicate the brain). Thus, an intracranial encephalopathy becomes the more important differential diagnosis and the most common of those is brain tumor in a 10 year old. The diagnosis of tumor requires MRI, however, a stressful and costly diagnostic of questionable value unless a caretaker is willing to pursue brain surgery should an operable tumor be discovered.
Other indications of cognitive dysfunction include disorientation, changes in social and interactive behavior - becoming "needier" or, conversely, more aloof - changes in locomotor and sleep cycle behaviors, and loss of housetraining. Sundowner's syndrome is seen in dogs and evidenced by nightime anxiety - vocalizing, aimless wandering, mainly - thought to arise because the sensory deprivation occurring in older dogs is magnified by the dark and quiet of nightime.
Unfortunately, this is a progressive disorder and often prompts euthanasia when owners are unable to control inappropriate behavior. If additional medical problems exist - brain tumor, e.g. - the onset of seizures may also prompt euthanasia. You may find that a prescription benzodiazepine such as alprazolam (Xanax) will be helpful for controlling his anxiety and pacing should these symptoms arise.
Ancillary care involves physically and mentally stimulating exercises such as swimming, massage, and range of motion exercises, encouraging relaxation, ensuring that he is taken out frequently to minimize the cost of elimination "mistakes", encouraging reestablishment of daily cycles by feeding at regular hours and at least a few hours before bedtime, and administering the alprazolam before bed. Specialized diets rich in antioxidants may be of value such as Hill's Prescription Diet b/d. The monoamine oxidase inhibitor selegiline (Anipryl) is the only drug licensed for use for the treatment of canine cognitive dysfunction in the States. Many of us aren't impressed with the studies supporting its use, however.
Cognitive dysfunction in dogs is just as difficult to manage as is Altzheimer's in humans. Please respond with further questions or concerns if you wish.