A reduction in the muscle mass (muscle atrophy) may occur either due to disuse or a disease. Lack of physical activity due to sedentary life style or prolonged bed rest may result in a loss of muscle tone and mass. This can usually be reversed by a planned exercise regimen. Diseases which cause atrophy maybe either an affliction of the muscle (muscular dystrophy, multiple sclerosis and various myopathies) or damage to the nerve supplying the muscle or muscle group like polio, motor neuron disease, inflammatory polyneuropathy, entrapment syndromes, peripheral vascular disease, tumours, disc prolapse, syringomyelia, etc.
It is difficult to comment without a physical examination as one needs to know whether it is focal or diffuse, associated sensory changes and presence/absence of other signs. Please consult a neurologist who can examine you. You will also need nerve conduction study, EMG and probably a CT scan or MRI along with some other tests. Application of moist heat and active resistive exercises may help.