AC or DC??: My guess is that it has to do with the small joint where the collar bone meets the shoulder bone.That "lump" may represent new bone growth in response to the inflammation there. You're kinda young to have this unless you play lots of tennis, lift weights, or injured that area before...things like that). It's called the "acromioclavicular joint, or A-C joint, It can have arthritis and not hurt much
Answered 11/15/2015
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