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Arthritic changes can be pretty painful for dogs but there are some great anti-inflammatories that your vet can prescribe that will help a great deal and they are not terribly expensive.
Keep in mind that there is also a disorder called vestibular disease that affects dogs. It's a disease of the inner ear and it causes stroke-like symptoms. If your dog is stiff or unable to move one side, this might be the cause.
No matter what, you need to see the vet as soon as you can.
Let me know if you still have questions.