It only took nearly 2 months, but a Dr. finally figured out why my arm swelled. I have polymyositis.

Polymyositis is an auto-immune disorder like Lupus or Rheumatoid arthritis.

To get it under control, I have to take Prednisone--eventually the MD will taper me off of the steroids, and get me on some sort of maintenance medication.
Oh, and stress only makes the symptoms worse....
But, at least I have a diagnosis to work with.

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Mental that one, I'm telling you. ---Ron Weasley, "Harry Potter and the Chamber of Secrets"