can occur due to friction with tight clothing(as mentioned by you )
Even the use of deodourants and antiperspirants can cause it
waxing .shaving,or hair removal
creams can cause it
it can be post inflamatory after a bacterial or a fungal infection
To treat this
peeling- tca peels or glycolic or mixed peels can be done
apply depigmenting creams like kojic acid or glycolic acid
or azaelic acid twice a day under dermat's guidance as there are chances of irritation with these especially in folds like axillary area.
deodourants and sprays should be avoided.