Smaller dogs can have problems with the discs in the spine and this can happen in the neck area or the lower spine. It can cause intermittent limping to paralysis. The pain can range from mild to severe. What happens is the soft tissue that cushions the dis becomes hardened and presses on the spinal cord. This can happen over time or suddenly. Treatment is anti inflammatory drugs and cage rest to surgery. Please note that a regular x ray may not pick this up, a myelogram should be done.
I am going to direct you to a site regarding this for further reading. I will include both the neck area and the spinal area.
Lower spine: http://www.petplace.com/dogs/intervertebral-disc-disease-thoracolumbar-area-in-dogs/page1.aspx
Pain can cause shaking however I would take the dog's temperature to make sure that is normal. Normal for dogs is 100.5 -102.5. Take it rectally for the best results. A temp may indicate infection. http://www.petplace.com/dogs/how-to-take-your-dog-s-temperature/page1.aspx
I suspect a disc problem as this is a prone illness to the breed which will cause intermittent limping. I think that old age would show more of a problem with the hind legs. But I could concede to see how you would think that and I would not rule it out. Dogs do get arthritis and you may be looking at that. Rimadyl is usually prescribed for this.
You may want to try giving the dog aspirin and see if that helps. Make sure to use aspirin and not Tylenol, Ibuprofen or acetaminophen as these are toxic to dogs. Here is information and a dosing chart. Also this can only be used 2-3 days at best or stomach ulcers can occur. http://www.petplace.com/drug-library/acetylsalicylic-acid-aspirin/page1.aspx
Here is a list of prone illnesses to this breed http://www.petplace.com/dogs/choosing-an-american-cocker-spaniel/page3.aspx