Why does my left armpit smell and not my right one ?

is it normal? can it be treated?

It could be thoracic outlet syndrome or something else to do with the nerve supply via the sympathetic nervous system. Shoulder muscles are probably squeezing a nerve. If that's what it is, you may be able to treat it yourself by shrugging your shoulders back and forth and it's exacerbated by poor posture, so you might want to look into Alexander Technique, a method of improving use of the body without being conscious of it. You could also try hot and cold packs.

See a health care professional if it concerns you.