Shaking or trembling in toy breeds at a young age could be due to hypoglycemia, liver shunt, or a neurologic response such as a seizure. Trembling can also be a sign of pain, stress and cold.
Hypoglycemia is low blood sugar and frequently causing trembling in small breeds. Feeding your dog smaller meals more frequently can help. If you suspect your dog is having low blood sugar you can put a drop of pancake syrup on your dog's tongue which should raise the level.
A liver shunt is usually a genetic condition. It is a condition where instead of the blood going through the liver and being cleansed, part of the blood is diverted around the liver resulting in a toxic buildup in the blood. You can read about these here: http://www.malteseonly.com/shunt2.html
Seizures can manifest as trembling and a dog may not lose conscienceness. You can read more about seizures here: http://www.marvistavet.com/html/body_seizure_disorder.html
Many different things can cause constant panting. Heavy panting can be caused by heart problems, lung problems, fever, or bronchitis. You can read about this here:
Dogs pant for various reasons. They pant to cool down, but they also pant when they are under stress, in pain, or afraid. There can also be medical causes such as neurological problems, respiratory disorders, and Heart problems such as heartworm, anemia, and fever.
Given your dog's breed and age, I would suspect pain is the issue and possibly related to a back or hip problem, though it could be numerous other possibilities.
Your dog may have injured her back or have a problem with an invertebral disc. An intervertebral disc that has slipped or ruptured up into the spinal canal causes inflammation of the spinal cord, which in severe cases causes paralyses of the rear legs. You can read about this here:
Large dog breeds sometimes suffer from a condition called degenerative myelopathy which can also cause problems with the rear legs and paralyses and pain. You can read about this here:
Buffered aspirin can be given to a dog with a dosage of up to 5-10 mg per pound every 12 hours for pain. If this seems to relieve the symptoms, then you should have her seen to discover the cause of the pain. Keep in mind that a dog's body does not metabolize aspirin in the same way as a human and thus should not be given more than a day or two without contacting your Vet. Read side effects and precautions here.
You will want to have your dog seen by your Vet to be evaluated and discuss treatment options.
Hope this helps.