HI,Welcome with your query to HCM.
Based on the facts of your query,after its review,In my opinion you seem to have Sarcoid lympnode in the armpit/crease,with compression symptoms lower down in that arm.
You need to consult Physician and if conservative treatment with steroid
does not regress the lower arm compression symptoms,
then elective surgery
to decompress armpit/crease by Excision of the Sarcoid Adenitis
lump in armcrease would resolve your pains in that arm.
Hope this reply resolves your worry with your queries.
Will appreciate writing your feedback review comments,to help the needy patients like you at HCM.
Senior Surgical Specialist