Neck and spinal Arthritis problems are very common in older dogs, causing pain and neurological symptoms.
Use a harness instead of a collar to reduce traction on her neck. Provide a comfortable bed low on the floor so she won't need to try and jump up. Also you may want to have a heating pad on low under her bed to provide gentle heat (always keep a couple layers of bedding between her and the pad - don't allow her to be directly on the pad). Carry her over steps and lift/lower her to avoid having her jump on your bed, sofa, or other such areas.
Metacam (pain reliever and antinflamatory) may potentially react with supplement ingredients. Please discuss any supplement use with your veterinarian before starting something new.