The temporal relationship of the injection quickly followed by painless muscle atrophy suggests a cause and effect relationship between the two. Fortunately, such atrophy following deltoid injection is most uncommon. However, atrophy of one or more (of the 3) heads of the deltoid muscle, following an intramuscular injection of that muscle has been described in the medical literature.* It has been postulated that the atrophy is secondary to injury to the axillary nerve, either from the needle and/or the material injected. The same investigators suggested that the presence of voluntary motor units (nerves) in the middle head of the deltoid muscle, might “portent a favorable prognosis” for continued re-innervation of the other two muscular “heads.”
The best advice I can give is that you ask your doctor to refer you to a medical specialist in Physical Medicine, called a Physiatrist, who can perform diagnostic studies (nerve and muscle function) to further define the presence and extent of muscle atrophy and recommend appropriate physical therapy, in response to which muscle function and size may or may not respond.
Good luck,
* Davidson LT, Carter GT, Kilmer DD, Han JJ: Iatrogenic axillary neuropathy after intramuscular
injection of the deltoid muscle. Am J Phys Med Rehabil 2007;86:507–511.