HI lotus flower,
Smaller dogs are prone to disc problems such as intervertebral disc disease and this is when the soft tissue that cushions the disc suddenly turns hard and presses on the spinal cord. This will affect him when trying to jump up on furniture and it may get worse to where you see him limping, having problems getting up and down steps or yelping when touched.
Generally anti inflammatory meds and strict rest are recommended. I would have your vet give an examination and something for pain.
You can read more on disc disease here. You may have to close a pop up window to read the article. http://www.petplace.com/dogs/intervertebral-disc-disease-thoracolumbar-area-in-dogs/page1.aspx
dog training consultant
24 years training & behavior,pet first aide, Therapy dog eval., PA Animal Response Team